Driver Circuit, Display Device, And Electronic Device

ABSTRACT

To suppress malfunctions in a shift register circuit. A shift register having a plurality of flip-flop circuits is provided. The flip-flop circuit includes a transistor  11 , a transistor  12 , a transistor  13 , a transistor  14 , and a transistor  15 . When the transistor  13  or the transistor  14  is turned on in a non-selection period, the potential of a node A is set, so that the node A is prevented from entering into a floating state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/675,077, filed Nov. 13, 2012, now allowed, which is a continuation ofU.S. application Ser. No. 12/477,338, filed Jun. 3, 2009, now U.S. Pat.No. 8,314,765, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2008-157400 on Jun. 17, 2008,all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driver circuit. In particular, thepresent invention relates to a display device having the driver circuit.Further, the present invention relates to an electronic device havingthe display device in a display portion.

2. Description of the Related Art

In recent years, with the increase of large display devices such asliquid crystal televisions, display devices such as liquid crystaldisplay devices and light-emitting devices have been actively developed.In particular, a technique for forming a pixel circuit and a drivercircuit including a shift register or the like (also referred to as aninternal circuit) over the same insulating substrate by usingtransistors or the like having semiconductor layers has been activelydeveloped, because the technique greatly contributes to reduction inpower consumption and cost. The internal circuit formed over theinsulating substrate is connected to an external circuit including acontroller IC or the like provided outside the insulating substratethrough an FPC or the like, and its operation is controlled.

As a driver circuit (also referred to as a driver), which is one ofinternal circuits, there is a scan line driver circuit or the like, forexample. For example, a driver circuit is formed using a shift registerincluding a plurality of flip-flop circuits, as disclosed in Reference1.

REFERENCE

Reference 1: Japanese Published Patent Application No. 2006-024350

SUMMARY OF THE INVENTION

In a conventional driver circuit as disclosed in Reference 1, there is aproblem in that malfunctions occur because timing of the switchingoperation of a transistor in a flip-flop circuit deviates from desiredtiming. As a cause of deviation in timing of the switching operation ofa transistor, for example, when a gate terminal of a pull-up transistorin a flip-flop circuit of a shift register enters into a floating statein a non-selection period, noise or the like generated in thenon-selection period adversely affects a potential of the gate terminalof the pull-up transistor.

In addition, deterioration of a transistor itself is one of causes ofdeviation in the timing of the switching operation. Due to thedeterioration of the transistor, the threshold voltage of the transistorchanges, so that malfunctions occur in the driver circuit. In the caseof using a transistor having a semiconductor layer formed using anamorphous semiconductor as a transistor, malfunctions particularly occureasily because the transistor having the semiconductor layer formedusing the amorphous semiconductor easily deteriorates.

In an embodiment of the present invention, it is an object to suppressmalfunctions in a circuit including a shift register.

An embodiment of the present invention is a driver circuit whichincludes a shift register including a plurality of flip-flop circuits.At least one of the plurality of flip-flop circuits is a flip-flopcircuit to which a first signal, a second signal, and a third signal areinput and which outputs an output signal. The at least one of theplurality of flip-flop circuits includes a first transistor, a secondtransistor, a third transistor, a fourth transistor, and a fifthtransistor. The first transistor includes a gate terminal, a sourceterminal, and a drain terminal. A first potential corresponding to apotential of the first signal is applied to the gate terminal of thefirst transistor. The first potential or a second potential is appliedto one of the source terminal and the drain terminal of the firsttransistor. The second transistor includes a gate terminal, a sourceterminal, and a drain terminal. A third potential corresponding to apotential of the second signal is applied to the gate terminal of thesecond transistor. One of the source terminal and the drain terminal ofthe second transistor is electrically connected to the other of thesource terminal and the drain terminal of the first transistor. A fourthpotential is applied to the other of the source terminal and the drainterminal of the second transistor. One of the third transistor and thefourth transistor controls whether to set a potential of the other ofthe source terminal and the drain terminal of the first transistor tothe first potential or the fourth potential. The other of the thirdtransistor and the fourth transistor controls whether to set thepotential of the other of the source terminal and the drain terminal ofthe first transistor to the fourth potential. When the one of the thirdtransistor and the fourth transistor is in an on state, the other of thethird transistor and the fourth transistor is in an off state. When theother of the third transistor and the fourth transistor is in an onstate, the one of the third transistor and the fourth transistor is inan off state. The fifth transistor includes a gate terminal, a sourceterminal, and a drain terminal. The gate terminal of the fifthtransistor is electrically connected to the other of the source terminaland the drain terminal of the first transistor. A fifth potentialcorresponding to a potential of the third signal is applied to one ofthe source terminal and the drain terminal of the fifth transistor. Apotential of the other of the source terminal and the drain terminal ofthe fifth transistor is a potential of the output signal. The fifthtransistor is in an off state when the third transistor or the fourthtransistor is in an on state.

An embodiment of the present invention is a driver circuit whichincludes a shift register including a plurality of flip-flop circuits.The flip-flop circuit is a flip-flop circuit to which a first controlsignal, a second control signal, a first clock signal, and a secondclock signal are input and which outputs an output signal. The flip-flopcircuit includes a first transistor, a second transistor, a thirdtransistor, a fourth transistor, a fifth transistor, a sixth transistor,and a seventh transistor. The first transistor includes a gate terminal,a source terminal, and a drain terminal. A first potential correspondingto a potential of the first control signal is applied to the gateterminal of the first transistor. The first potential or a secondpotential is applied to one of the source terminal and the drainterminal of the first transistor. The second transistor includes a gateterminal, a source terminal, and a drain terminal. A third potentialcorresponding to a potential of the second control signal is applied tothe gate terminal of the second transistor. One of the source terminaland the drain terminal of the second transistor is electricallyconnected to the other of the source terminal and the drain terminal ofthe first transistor. A fourth potential is applied to the other of thesource terminal and the drain terminal of the second transistor. Each ofthe third transistor and the fourth transistor is a transistor includinga gate terminal, a source terminal, and a drain terminal. One of thesource terminal and the drain terminal of the third transistor iselectrically connected to the other of the source terminal and the drainterminal of the first transistor. One of the source terminal and thedrain terminal of the fourth transistor is electrically connected to theother of the source terminal and the drain terminal of the firsttransistor. The first potential or the fourth potential is applied tothe other of the source terminal and the drain terminal of the one ofthe third transistor and the fourth transistor. One of the sourceterminal and the drain terminal of the other of the third transistor andthe fourth transistor is electrically connected to the other of thesource terminal and the drain terminal of the first transistor. Thefourth potential is applied to the other of the source terminal and thedrain terminal of the other of the third transistor and the fourthtransistor. When the one of the third transistor and the fourthtransistor is in an on state, the other of the third transistor and thefourth transistor is turned off. When the other of the third transistorand the fourth transistor is in an on state, the one of the thirdtransistor and the fourth transistor is turned off. The fifth transistorincludes a gate terminal, a source terminal, and a drain terminal. Thegate terminal of the fifth transistor is electrically connected to theother of the source terminal and the drain terminal of the firsttransistor. A fifth potential corresponding to a potential of the firstclock signal is applied to one of the source terminal and the drainterminal of the fifth transistor. A potential of the other of the sourceterminal and the drain terminal of the fifth transistor is a potentialof the output signal. The fifth transistor is in an off state when thethird transistor or the fourth transistor is in an on state. The sixthtransistor includes a gate terminal, a source terminal, and a drainterminal. The gate terminal of the sixth transistor is electricallyconnected to the gate terminal of the other of the third transistor andthe fourth transistor. One of the source terminal and the drain terminalof the sixth transistor is electrically connected to the other of thesource terminal and the drain terminal of the fifth transistor. Thefourth potential is applied to the other of the source terminal and thedrain terminal of the sixth transistor. The seventh transistor includesa gate terminal, a source terminal, and a drain terminal. A sixthpotential corresponding to a potential of the second clock signal isapplied to the gate terminal of the seventh transistor. One of thesource terminal and the drain terminal of the seventh transistor iselectrically connected to the other of the source terminal and the drainterminal of the fifth transistor. The fourth potential is applied to theother of the source terminal and the drain terminal of the seventhtransistor.

Note that in the above embodiment of the present invention, theflip-flop circuit can include a first capacitor, an eighth transistor, asecond capacitor, and a ninth transistor. The first capacitor includesat least two terminals. The fifth potential is applied to one of theterminals of the first capacitor. The other of the terminals of thefirst capacitor is electrically connected to the gate terminal of theother of the third transistor and the fourth transistor. The eighthtransistor includes a gate terminal, a source terminal, and a drainterminal. The gate terminal of the eighth transistor is electricallyconnected to the gate terminal of the fifth transistor. One of thesource terminal and the drain terminal of the eighth transistor iselectrically connected to the gate terminal of the other of the thirdtransistor and the fourth transistor. The fourth potential is applied tothe other of the source terminal and the drain terminal of the eighthtransistor. The second capacitor includes at least two terminals. Thesixth potential is applied to one of the terminals of the secondcapacitor. The other of the terminals of the second capacitor iselectrically connected to the gate terminal of the one of the thirdtransistor and the fourth transistor. The ninth transistor includes agate terminal, a source terminal, and a drain terminal. The gateterminal of the ninth transistor is electrically connected to the gateterminal of the first transistor. One of the source terminal and thedrain terminal of the ninth transistor is electrically connected to thegate terminal of the one of the third transistor and the fourthtransistor. The fourth potential is applied to the other of the sourceterminal and the drain terminal of the ninth transistor.

In the above embodiment of the present invention, the flip-flop circuitcan include a first capacitor and an eighth transistor. The firstcapacitor includes at least two terminals. The fifth potential isapplied to one of the terminals of the first capacitor. The other of theterminals of the first capacitor is electrically connected to the gateterminal of the other of the third transistor and the fourth transistor.The eighth transistor includes a gate terminal, a source terminal, and adrain terminal. The gate terminal of the eighth transistor iselectrically connected to the gate terminal of the fifth transistor. Oneof the source terminal and the drain terminal of the eighth transistoris electrically connected to the gate terminal of the other of the thirdtransistor and the fourth transistor. The fourth potential is applied tothe other of the source terminal and the drain terminal of the eighthtransistor.

In the above embodiment of the present invention, the flip-flop circuitcan include a tenth transistor. The tenth transistor includes a gateterminal, a source terminal, and a drain terminal. The first potentialis applied to the gate terminal of the tenth transistor. One of thesource terminal and the drain terminal of the tenth transistor iselectrically connected to the gate terminal of the other of the thirdtransistor and the fourth transistor. The fourth potential is applied tothe other of the source terminal and the drain terminal of the tenthtransistor.

In the above embodiment of the present invention, the flip-flop circuitcan have a function of outputting a second output signal and can includean eleventh transistor, a twelfth transistor, and a thirteenthtransistor. The eleventh transistor includes a gate terminal, a sourceterminal, and a drain terminal. The gate terminal of the eleventhtransistor is electrically connected to the one of the source terminaland the drain terminal of the first transistor. The fifth potential isapplied to one of the source terminal and the drain terminal of theeleventh transistor. A potential of the other of the source terminal andthe drain terminal of the eleventh transistor is a potential of thesecond output signal. The twelfth transistor includes a gate terminal, asource terminal, and a drain terminal. The gate terminal of the twelfthtransistor is electrically connected to the gate terminal of the otherof the third transistor and the fourth transistor. One of the sourceterminal and the drain terminal of the twelfth transistor iselectrically connected to the other of the source terminal and the drainterminal of the eleventh transistor. The fourth potential is applied tothe other of the source terminal and the drain terminal of the twelfthtransistor. The thirteenth transistor includes a gate terminal, a sourceterminal, and a drain terminal. The gate terminal of the thirteenthtransistor is electrically connected to the gate terminal of the seventhtransistor. One of the source terminal and the drain terminal of thethirteenth transistor is electrically connected to the other of thesource terminal and the drain terminal of the eleventh transistor. Thefourth potential is applied to the other of the source terminal and thedrain terminal of the thirteenth transistor.

In the above embodiment of the present invention, the first controlsignal and the second control signal are digital signals, and theabsolute value of a potential difference between a high state and a lowstate of each digital signal can be made larger than the absolute valueof the threshold voltage of each transistor in the flip-flop circuit.

In the above embodiment of the present invention, the level of thefourth potential can be made equivalent to the level of a potential of ahigh state or a low state of the first control signal, the secondcontrol signal, the first clock signal, or the second clock signal.

In the above embodiment of the present invention, the phase of the firstclock signal and the phase of the second clock signal are opposite toeach other, and the absolute value of a potential difference between ahigh state and a low state of each of the first clock signal and thesecond clock signal can be made larger than the absolute value of thethreshold voltage of each transistor in the flip-flop circuit.

In the above embodiment of the present invention, all the transistors inthe flip-flop circuit can have the same conductivity type.

In the above embodiment of the present invention, each transistor in theflip-flop circuit can include a gate electrode, a gate insulating filmprovided so as to cover the gate electrode, a first semiconductor layerincluding a microcrystalline semiconductor layer and provided over thegate electrode with the gate insulating film interposed therebetween, abuffer layer provided over the first semiconductor layer, a pair ofsecond semiconductor layers including an impurity element and providedover the buffer layer, a source electrode provided over one of the pairof second semiconductor layers, and a drain electrode provided over theother of the pair of second semiconductor layers.

An embodiment of the present invention is a display device whichincludes one of a scan line driver circuit and a signal line drivercircuit having the above driver circuit, a plurality of scan lines, aplurality of signal lines, and a pixel portion. The pixel portionincludes a plurality of pixels which are electrically connected to thescan line driver circuit through any one of the plurality of scan linesand are electrically connected to the signal line driver circuit throughany one of the plurality of signal lines.

An embodiment of the present invention is an electronic device havingthe above display device in a display portion.

Note that in this specification, a transistor has at least threeterminals: a gate terminal, a drain terminal, and a source terminal. Agate terminal refers to part of a gate electrode (including a conductivefilm, a wiring, and the like) or part of a portion which is electricallyconnected to the gate electrode. In addition, a source terminal refersto part of a source electrode (including a conductive layer, a wiring,and the like) or part of a portion which is electrically connected tothe source electrode. Further, a drain terminal refers to part of adrain electrode (including a conductive layer, a wiring, and the like)or part of a portion which is electrically connected to the drainelectrode. Furthermore, the transistor has a channel region between adrain region and a source region and can supply current through thedrain region, the channel region, and the source region.

Further, in this specification, since a source terminal and a drainterminal of a transistor change depending on the structure, theoperating condition, or the like of the transistor, it is difficult todefine which is a source terminal or a drain terminal. Therefore, inthis document, one of terminals selected optionally from a sourceterminal and a drain terminal is referred to one of the source terminaland the drain terminal, and the other of the terminals is referred to asthe other of the source terminal and the drain terminal.

Note that when it is explicitly described that “B is formed on A” or “Bis formed over A”, it does not necessarily mean that B is formed indirect contact with A. The description includes the case where A and Bare not in direct contact with each other, i.e., the case where anotherobject is interposed between A and B. Here, each of A and B correspondsto an object (e.g., a device, an element, a circuit, a wiring, anelectrode, a terminal, a conductive film, or a layer).

Therefore, for example, when it is explicitly described that “a layer Bis formed on (or over) a layer A”, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or D. Note that another layer (e.g., a layer C or a layer D) maybe a single layer or a plurality of layers.

Note that when it is explicitly described that “B is formed on A” or “Bis formed over A”, it includes the case where B is formed obliquelyabove A.

Further, in this specification, terms with ordinal numbers, such as“first” and “second”, are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

According to an embodiment of the present invention, malfunctions in acircuit including a shift register can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram illustrating an example of the structure ofa driver circuit in Embodiment 1;

FIG. 2 is a timing chart illustrating the operation of the drivercircuit illustrated in FIG. 1;

FIG. 3 is a circuit diagram illustrating an example of the structure ofthe driver circuit in Embodiment 1;

FIG. 4 is a circuit diagram illustrating an example of the structure ofthe driver circuit in Embodiment 1;

FIG. 5 is a timing chart illustrating the operation of the drivercircuit illustrated in FIG. 4;

FIG. 6 is a circuit diagram illustrating an example of the structure ofthe driver circuit in Embodiment 1;

FIG. 7 is a circuit diagram illustrating an example of the structure ofa driver circuit in Embodiment 2;

FIG. 8 is a timing chart illustrating the operation of the drivercircuit illustrated in FIG. 7;

FIG. 9 is a circuit diagram illustrating an example of the structure ofthe driver circuit in Embodiment 2;

FIG. 10 is a circuit diagram illustrating an example of the structure ofthe driver circuit in Embodiment 2;

FIG. 11 is a timing chart illustrating the operation of the drivercircuit illustrated in FIG. 10;

FIG. 12 is a circuit diagram illustrating an example of the structure ofthe driver circuit in Embodiment 2;

FIG. 13 is a circuit diagram illustrating an example of the structure ofa display device in Embodiment 3;

FIG. 14 is a timing chart illustrating the operation of a scan linedriver circuit 702 illustrated in FIG. 13;

FIGS. 15A to 15G illustrate examples of the structure and the operationof a pixel in a liquid crystal display device of Embodiment 3;

FIGS. 16A to 16H illustrate examples of the structure and the operationof a pixel in the liquid crystal display device of Embodiment 3;

FIGS. 17A and 17B are cross-sectional schematic views each illustratingan example of the structure of a transistor which can be used for adriver circuit in Embodiment 4;

FIG. 18 is a cross-sectional schematic view illustrating an example ofthe structure of a transistor which can be used for the driver circuitin Embodiment 4;

FIGS. 19A to 19C are cross-sectional schematic views illustrating anexample of a method for manufacturing a transistor which can be used forthe driver circuit in Embodiment 4;

FIGS. 20D to 20F are cross-sectional schematic views illustrating theexample of the method for manufacturing a transistor which can be usedfor the driver circuit in Embodiment 4;

FIGS. 21G and 21H are cross-sectional schematic views illustrating theexample of the method for manufacturing a transistor which can be usedfor the driver circuit in Embodiment 4;

FIGS. 22A to 22H each illustrate an example of an electronic device inwhich a display device in Embodiment 5 can be used for a displayportion;

FIGS. 23A to 23C illustrate an example of an electronic device in whichthe display device in Embodiment 5 can be used for a display portion;

FIG. 24 is a circuit diagram illustrating an example of the structure ofthe driver circuit in Embodiment 1; and

FIGS. 25A and 25B are graphs each illustrating the results of circuitsimulation of the driver circuit in Embodiment 1.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, examples of embodiments of the present invention will bedescribed with reference to the drawings. Note that the presentinvention is not limited to the following description. The presentinvention can be implemented in various different ways and it will bereadily appreciated by those skilled in the art that various changes andmodifications are possible without departing from the spirit and scopeof the present invention. Therefore, the present invention should not beconstrued as being limited to the following description of theembodiments.

Embodiment 1

In this embodiment, a driver circuit which is an embodiment of thepresent invention is described.

A driver circuit in this embodiment includes a shift register includinga plurality of flip-flop circuits.

An example of the circuit structure of the flip-flop circuit isdescribed with reference to FIG. 24. FIG. 24 is a circuit diagramillustrating an example of the circuit structure of the flip-flopcircuit in the driver circuit of this embodiment.

At least one of the plurality of flip-flop circuits can be a flip-flopcircuit having the circuit structure illustrated in FIG. 24. Note thatthe flip-flop circuit illustrated in FIG. 24 is, for example, a circuitto which a first signal, a second signal, and a third signal are inputand which has a function of outputting an output signal.

The flip-flop circuit illustrated in FIG. 24 includes a transistor 11, atransistor 12, a transistor 13, a transistor 14, and a transistor 15.

A first potential corresponding to a potential of the first signal isapplied to a gate terminal of the transistor 11. The first potential ora second potential is applied to one of a source terminal and a drainterminal of the transistor 11.

One of a source terminal and a drain terminal of the transistor 12 iselectrically connected to the other of the source terminal and the drainterminal of the transistor 11. In addition, a third potentialcorresponding to a potential of the second signal is applied to a gateterminal of the transistor 12. A fourth potential is applied to theother of the source terminal and the drain terminal of the transistor12.

One of the transistor 13 and the transistor 14 has a function ofcontrolling whether to set a potential of the other of the sourceterminal and the drain terminal of the transistor 11 to the firstpotential or the fourth potential. The other of the transistor 13 andthe transistor 14 has a function of controlling whether to set thepotential of the other of the source terminal and the drain terminal ofthe transistor 11 to the fourth potential.

When the transistor 14 is on, the transistor 13 has a function ofentering into an off state. Further, when the transistor 13 is in an onstate, the transistor 14 has a function of entering into an off state.

The gate terminal of the transistor 15 is electrically connected to theother of the source terminal and the drain terminal of the transistor11. In addition, a fifth potential corresponding to a potential of thethird signal is applied to one of a source terminal and a drain terminalof the transistor 15. A potential of the other of the source terminaland the drain terminal of the transistor 15 is a potential of the outputsignal. Note that a portion where the other of the source terminal andthe drain terminal of the transistor 11 and the gate terminal of thetransistor 15 are connected to each other is also referred to as a nodeA.

Further, the transistor 15 is in an off state when the transistor 13 orthe transistor 14 is in an on state.

With the above structure, when the transistor 13 or the transistor 14 ison, a potential of the node A, i.e., the potential of the gate terminalof the transistor 15 is set to a predetermined level, so that the node Adoes not enter into a floating state. Thus, malfunctions of theflip-flop circuit can be suppressed.

In addition, an example of the circuit structure of the flip-flopcircuit in the driver circuit of this embodiment is described withreference to FIG. 1. FIG. 1 is a circuit diagram illustrating an exampleof the circuit structure of the flip-flop circuit in this embodiment.

Each of the plurality of flip-flop circuits in the driver circuit ofthis embodiment can be a flip-flop circuit having the circuit structureillustrated in FIG. 1. The flip-flop circuit illustrated in FIG. 1includes a terminal 100, a terminal 101, a terminal 102, a terminal 103,a terminal 104, a terminal 105, a transistor 106, a transistor 107, acapacitor 108, a transistor 109, a transistor 110, a transistor 111, acapacitor 112, a transistor 113, a transistor 114, a transistor 115, anda transistor 116.

Note that although a terminal 102A and a terminal 102B are illustratedas the terminal 102 in the flip-flop circuit illustrated in FIG. 1, thestructure of the terminal 102 is not limited to this. In the flip-flopcircuit in the driver circuit of this embodiment, the terminal 102A andthe terminal 102B can be electrically connected to each other so as tobe one terminal 102. In addition, although a terminal 103A and aterminal 103B are illustrated as the terminal 103 in the flip-flopcircuit illustrated in FIG. 1, the structure of the terminal 103 is notlimited to this. In the flip-flop circuit in the driver circuit of thisembodiment, the terminal 103A and the terminal 103B can be electricallyconnected to each other so as to be one terminal 103.

Further, although terminals 104A to 104G are illustrated as the terminal104 in the flip-flop circuit in the driver circuit of this embodiment,the structure of the terminal 104 is not limited to this. In theflip-flop circuit in the driver circuit of this embodiment, theterminals 104A to 104G can be electrically connected to each other so asto be one terminal 104.

A gate terminal of the transistor 106 is electrically connected to theterminal 100. One of a source terminal and a drain terminal of thetransistor 106 is electrically connected to the gate terminal of thetransistor 106.

A gate terminal of the transistor 107 is electrically connected to theterminal 101. One of a source terminal and a drain terminal of thetransistor 107 is electrically connected to the other of the sourceterminal and the drain terminal of the transistor 106. The other of thesource terminal and the drain terminal of the transistor 107 iselectrically connected to the terminal 104A. Note that although notillustrated for convenience, a structure where the transistor 107 is notprovided can be used in the flip-flop circuit in the driver circuit ofthis embodiment. By using the structure where the transistor 107 is notprovided, the circuit area can be made smaller.

The capacitor 108 includes at least two terminals. One of the terminalsof the capacitor 108 is electrically connected to the terminal 102A.

A gate terminal of the transistor 109 is electrically connected to theone of the source terminal and the drain terminal of the transistor 106.One of a source terminal and a drain terminal of the transistor 109 iselectrically connected to the other of the terminals of the capacitor108. The other of the source terminal and the drain terminal of thetransistor 109 is electrically connected to the terminal 104B.

A gate terminal of the transistor 110 is electrically connected to theone of the source terminal and the drain terminal of the transistor 109.One of a source terminal and a drain terminal of the transistor 110 iselectrically connected to the other of the source terminal and the drainterminal of the transistor 106. The other of the source terminal and thedrain terminal of the transistor 110 is electrically connected to theterminal 104C.

The capacitor 112 includes at least two terminals. One of the terminalsof the capacitor 112 is electrically connected to the terminal 103A.

A gate terminal of the transistor 111 is electrically connected to theother of the terminals of the capacitor 112. One of a source terminaland a drain terminal of the transistor 111 is electrically connected tothe other of the source terminal and the drain terminal of thetransistor 106. The other of the source terminal and the drain terminalof the transistor 111 is electrically connected to the terminal 104D.

A gate terminal of the transistor 113 is electrically connected to theother of the source terminal and the drain terminal of the transistor106. One of a source terminal and a drain terminal of the transistor 113is electrically connected to the gate terminal of the transistor 111.The other of the source terminal and the drain terminal of thetransistor 113 is electrically connected to the terminal 104E.

A gate terminal of the transistor 114 is electrically connected to theother of the source terminal and the drain terminal of the transistor106. One of a source terminal and a drain terminal of the transistor 114is electrically connected to the terminal 103B. The other of the sourceterminal and the drain terminal of the transistor 114 is electricallyconnected to the terminal 105. A potential of the other of the sourceterminal and the drain terminal of the transistor 114 is an outputsignal and is output through the terminal 105. Note that in theflip-flop circuit in the driver circuit of this embodiment, a capacitorcan be additionally provided between the gate terminal of the transistor114 and the other of the source terminal and the drain terminal of thetransistor 114.

A gate terminal of the transistor 115 is electrically connected to thegate terminal of the transistor 111. One of a source terminal and adrain terminal of the transistor 115 is electrically connected to theother of the source terminal and the drain terminal of the transistor114. The other of the source terminal and the drain terminal of thetransistor 115 is electrically connected to the terminal 104F.

A gate terminal of the transistor 116 is electrically connected to theterminal 102B. One of a source terminal and a drain terminal of thetransistor 116 is electrically connected to the other of the sourceterminal and the drain terminal of the transistor 114. The other of thesource terminal and the drain terminal of the transistor 116 iselectrically connected to the terminal 104G.

Note that a portion where the one of the source terminal and the drainterminal of the transistor 109 is connected to the other of theterminals of the capacitor 108 or the gate terminal of the transistor110 is also referred to as a node 118. In addition, a portion where theother of the source terminal and the drain terminal of the transistor106 is connected to the one of the source terminal and the drainterminal of the transistor 107, the one of the source terminal and thedrain terminal of the transistor 110, the one of the source terminal andthe drain terminal of the transistor 111, the gate terminal of thetransistor 113, or the gate terminal of the transistor 114 is alsoreferred to as a node 117. Further, a portion where the gate terminal ofthe transistor 111 is connected to the other of the terminals of thecapacitor 112, the one of the source terminal and the drain terminal ofthe transistor 113, or the gate terminal of the transistor 115 is alsoreferred to as a node 119.

In the flip-flop circuit illustrated in FIG. 1, a first control signalis input through the terminal 100, and a second control signal is inputthrough the terminal 101. As each of the first control signal and thesecond control signal, a digital signal having two states of a highstate and a low state can be used, for example. In the case of using thedigital signal, the first control signal or the second control signalhaving a predetermined potential is input as a first potential (alsoreferred to as V1) through the terminal 100 or the terminal 101 when thefirst control signal or the second control signal, which is input, is ina high state (also referred to as a high level); the first controlsignal or the second control signal having a potential which is lowerthan the predetermined potential in the high state is input as a secondpotential (also referred to as V2) through the terminal 100 or theterminal 101 when the first control signal or the second control signal,which is input, is in a low state (also referred to as a low level). Thelevels of the potentials in the high state and the low state can be setas appropriate considering the level of the threshold voltage of eachtransistor, or the like, for example. For example, the levels of thepotentials in the high state and the low state are preferably set sothat a potential difference between the high state and the low state islarger than the absolute value of the threshold voltage of eachtransistor in the flip-flop circuit.

In the flip-flop circuit illustrated in FIG. 1, a clock signal which isin a first phase (also referred to as a first clock signal or a CKsignal) or a clock signal which is in a second phase (also referred toas a second clock signal, a CKB signal, or a signal obtained byinverting the first clock signal) is input through the terminal 102(also referred to as the terminal 102A and the terminal 102B). Each ofthe first clock signal and the second clock signal has two potentialstates of a high state and a low state. A potential of each clock signalis the potential V1 when each clock signal is in a high state (alsoreferred to as a high level), and the potential of each clock signal isthe potential V2 when each clock signal is in a low state (also referredto as a low level). Note that the levels of the potentials of the firstclock signal and the second clock signal in a high state are preferablyequivalent to the levels of the potentials of the first control signaland the second control signal in the high state. The levels of thepotentials of the first clock signal and the second clock signal in alow state are preferably equivalent to the levels of the potentials ofthe first control signal and the second control signal in the low state.Further, the levels of the potentials in the high state and the lowstate can be set as appropriate considering the level of the thresholdvoltage of each transistor, or the like, for example. For example, thelevels of the potentials in the high state and the low state arepreferably set so that a potential difference between the high state andthe low state is larger than the absolute value of the threshold voltageof each transistor in the flip-flop circuit.

The phase of the first clock signal and the phase of the second clocksignal are opposite to each other. For example, in a predeterminedperiod, the second clock signal is in the low state when the first clocksignal is in the high state, and the second clock signal is in the highstate when the first clock signal is in the low state.

In the flip-flop circuit, the first clock signal or the second clocksignal is input through the terminal 103 (also referred to as theterminal 103A and the terminal 103B). Note that the phase of the clocksignal which is input through the terminal 102 and the phase of thesecond clock signal which is input through the terminal 103 are oppositeto each other. For example, the second clock signal is input through theterminal 103 in the case where the first clock signal is input throughthe terminal 102, and the first clock signal is input through theterminal 103 in the case where the second clock signal is input throughthe terminal 102.

A potential having a predetermined level is applied to the flip-flopcircuit illustrated in FIG. 1 through the terminal 104 (also referred toas the terminals 104A to 104G). In this case, the level of the potentialhaving the predetermined level can be set to V1 or V2, for example. Thatis, the level of the potential having the predetermined level can bemade equivalent to the level of a potential of a digital signal such asa clock signal or a control signal in a high state or a low state.

Note that although the one of the source terminal and the drain terminalof the transistor 106 is electrically connected to the terminal 100 inthe flip-flop circuit illustrated in FIG. 1, the structure of theflip-flop circuit is not limited to this. In the flip-flop circuit inthe driver circuit of this embodiment, the one of the source terminaland the drain terminal of the transistor 106 can be electricallyconnected to a power supply terminal separately so that the potential V1or the potential V2 can be applied.

The transistor 106 has a function of controlling conduction between theterminal 100 and the node 117 in accordance with a signal which is inputthrough the terminal 100.

The transistor 107 has a function of controlling conduction between theterminal 104A and the node 117 in accordance with a signal which isinput through the terminal 101. By bringing the terminal 104A and thenode 117 into conduction, a potential of the node 117 is set to V1 orV2.

The capacitor 108 has a function of changing a potential of the node 118by capacitive coupling in accordance with a signal which is inputthrough the terminal 102 (the terminal 102A). For example, the capacitor108 has a function of setting the potential of the node 118 to thepotential V1 by capacitive coupling in the case where the signal whichis input through the terminal 102 (the terminal 102A) is changed from alow state to a high state. On the other hand, the capacitor 108 has afunction of setting the potential of the node 118 to V1 or V2 bycapacitive coupling in the case where the signal which is input throughthe terminal 102 is changed from the high state to the low state.

The transistor 109 has a function of controlling conduction between theterminal 104B and the node 118 in accordance with the signal which isinput through the terminal 100. By bringing the terminal 104B and thenode 118 into conduction, the potential of the node 118 is set to V1 orV2.

The transistor 110 has a function of controlling conduction between theterminal 104C and the node 117 in accordance with the potential of thenode 118. By bringing the terminal 104C and the node 117 intoconduction, the potential of the node 117 is set to V1 or V2. Inaddition, the transistor 110 has a function of entering into an offstate when the transistor 111 is in an on state.

The transistor 111 has a function of controlling conduction between theterminal 104D and the node 117 in accordance with a potential of thenode 119. By bringing the terminal 104D and the node 117 intoconduction, the potential of the node 117 is set to V1 or V2. Inaddition, the transistor 111 has a function of entering into an offstate when the transistor 110 is on.

The capacitor 112 has a function of changing the potential of the node119 by capacitive coupling in accordance with a signal which is inputthrough the terminal 103A. For example, the capacitor 112 sets thepotential of the node 119 to V1 by capacitive coupling in the case wherethe signal which is input through the terminal 103A is changed from alow state to a high state. On the other hand, the capacitor 112 sets thepotential of the node 119 to V2 by capacitive coupling in the case wherethe signal which is input through the terminal 103A is changed from thehigh state to the low state.

The transistor 113 has a function of controlling conduction between theterminal 104E and the node 119. By bringing the terminal 104E and thenode 119 into conduction, the potential of the node 119 is set to V1 orV2.

The transistor 114 has a function of controlling conduction between theterminal 103B and the terminal 105 in accordance with the potential ofthe node 117. By bringing the terminal 103B and the terminal 105 intoconduction, the transistor 114 makes the level of a potential of asignal which is input through the terminal 103B equivalent to the levelof the potential of a signal which is output through the terminal 105.

Further, the transistor 114, for example, is an n-channel transistor andhas a function of raising the potential of the node 117 in accordancewith rise in the potential of a connection portion between thetransistor 114 and the terminal 105 when the signal which is inputthrough the terminal 103B is changed from the low state to the highstate in the case where the potential of the node 117 is V1. That is,the transistor 114 performs so-called bootstrap operation. Note that thebootstrap operation is often performed using parasitic capacitancebetween the gate terminal of the transistor 114 and the other of thesource terminal and the drain terminal of the transistor 114.

The transistor 115 has a function of controlling conduction between theterminal 104F and the terminal 105 in accordance with the potential ofthe node 119. By bringing the terminal 104F and the terminal 105 intoconduction, a potential of the signal which is output through theterminal 105 is set to V1 or V2.

The transistor 116 has a function of controlling conduction between theterminal 104G and the terminal 105 in accordance with a signal which isinput through the terminal 102B. By bringing the terminal 104G and theterminal 105 into conduction, the transistor 116 sets the potential ofthe signal which is output through the terminal 105 to V1 or V2.

Note that since all the transistors can have the same conductivity typein the driver circuit of this embodiment, manufacturing steps can besimplified. Therefore, manufacturing cost can be reduced and yield canbe improved. Further, a semiconductor device such as a large displaypanel can be easily manufactured. In the driver circuit of thisembodiment, all the transistors can be transistors having n-typeconductivity (also referred to as n-channel transistors) or transistorshaving p-type conductivity (also referred to as p-channel transistors).Note that description “the same” also corresponds to description“substantially the same”.

Next, the operation of the driver circuit illustrated in FIG. 1 isdescribed with reference to FIG. 2. FIG. 2 is a timing chartillustrating an example of the operation of the driver circuitillustrated in FIG. 1. Note that in this embodiment, as an example, thesecond clock signal is input through the terminal 102 and the firstclock signal is input through the terminal 103. In addition, here, as anexample of the operation of the driver circuit illustrated in FIG. 1,the case where all the transistors in the flip-flop circuit aren-channel transistors is described.

As for the operation of the driver circuit illustrated in FIG. 1,predetermined operation in a certain period is repeated, as illustratedin FIG. 2. The certain period is divided into a selection period and anon-selection period. Further, the selection period and thenon-selection period are divided into a first period, a second period, athird period, a fourth period, and a fifth period. In FIG. 2, the firstperiod, the third period, the fourth period, and the fifth period arethe non-selection period, and the second period is the selection period.

First, in the first period, a first control signal 201 which is in ahigh state is input through the terminal 100; a second control signal208 which is in a low state is input through the terminal 101; a secondclock signal 203 which is in a high state is input through the terminal102; a first clock signal 202 which is in a low state is input throughthe terminal 103. Accordingly, the transistor 106, the transistor 109,and the transistor 116 are turned on, and the transistor 107 is turnedoff.

When the transistor 106 is turned on, a potential 204 of the node 117starts to rise. In this case, the potential of the node 117 rises to avalue obtained by subtracting the threshold voltage of the transistor106 (also referred to as Vth₁₀₆) from the potential V1 of the firstcontrol signal 201, i.e., to V1-Vth₁₀₆. When the potential of the node117 is at V1-Vth₁₀₆, the transistor 106 is turned off.

When the potential 204 of the node 117 is at V1-Vth₁₀₆, the transistor113 is turned on. In this case, the level of a potential 206 of the node119 becomes equivalent to the level of the potential V2 which is appliedthrough the terminal 104E.

When the potential 206 of the node 119 is at V2, the transistor 111 andthe transistor 115 are turned off.

When the transistor 109 is turned on, the level of a potential 205 ofthe node 118 becomes equivalent to the level of the potential V2 whichis applied through the terminal 104B.

When the potential 205 of the node 118 is at V2, the transistor 110 isturned off.

When the transistor 106, the transistor 107, the transistor 110, and thetransistor 111 are turned off as described above, the node 117 entersinto a floating state with the potential thereof kept at V1-Vth₁₀₆.

When the potential 204 of the node 117 is at V1-Vth₁₀₆, the transistor114 is turned on.

In this case, the level of a potential of an output signal 207 which isoutput through the terminal 105 becomes equivalent to the level of thepotential V2 which is applied through the terminal 103B or the level ofthe potential V2 which is applied through the terminal 104G. The aboveis the operation in the first period.

Next, in the second period, the first control signal 201 which is in alow state is input through the terminal 100; the second control signal208 which is in the low state is input through the terminal 101; thesecond control signal 203 which is in a low state is input through theterminal 102; the first control signal 202 which is in a high state isinput through the terminal 103. In this case, the transistor 106, thetransistor 109, and the transistor 116 are turned off, and thetransistor 107 is kept off.

Note that the transistor 109 is turned off after the second clock signal203 which is input through the terminal 102A enters into a low state inmany cases. This is because the first control signal 201 which is inputthrough the terminal 100 is often delayed as compared to the secondclock signal 203. By turning off the transistor 109 after the secondclock signal 203 enters into the low state, the node 118 enters into afloating state with the potential thereof kept at the potential V2, andthe transistor 110 is kept off.

The capacitor 108 holds a potential difference between a potential ofthe second clock signal 203 which is input through the terminal 102A andthe potential 205 of the node 118, i.e., a potential difference betweenthe potential of the second clock signal 203 which is in the low stateand the potential V2 which is applied through the terminal 104B.

When the transistor 106, the transistor 107, and the transistor 110 arein an off state as described above, the potential 204 of the node 117 iskept at V1-Vth₁₀₆.

When the potential 204 of the node 117 is V1-Vth₁₀₆, the transistor 113is kept on. When the transistor 113 is kept on, the potential 206 of thenode 119 is kept at V2, and the transistor 111 and the transistor 115are kept off.

When the potential 204 of the node 117 is kept at V1-Vth₁₀₆ and apotential of the one of the source terminal and the drain terminal ofthe transistor 114 is at the potential V1 of the first clock signal 202,a potential of the output signal 207 which is output through theterminal 105 rises. Then, since the node 117 is in a floating state, thepotential 204 of the node 117 rises by capacitive coupling of parasiticcapacitance between the gate terminal of the transistor 114 and theother of the source terminal and the drain terminal of the transistor114 in accordance with the potential of the output signal 207. This isso-called bootstrap operation.

The potential 204 of the node 117 rises to a value which is larger thanthe sum of the potential V1 of the first clock signal 202 and thethreshold voltage of the transistor 114 (also referred to as Vth₁₁₄),i.e., to V1+Vth₁₁₄+Va (Va is a given positive number). In this case, thetransistor 114 is kept on.

In this case, the level of the potential of the output signal 207 whichis output through the terminal 105 becomes equivalent to the level ofthe potential V1 which is applied through the terminal 103B. The aboveis the operation in the second period.

Next, in the third period, the first control signal 201 which is in thelow state is input through the terminal 100; the second control signal208 which is in a high state is input through the terminal 101; thesecond clock signal 203 which is in the high state is input through theterminal 102; the first clock signal 202 which is in the low state isinput through the terminal 103. In this case, the transistor 107 and thetransistor 116 are turned on, and the transistor 106 and the transistor109 are kept off.

When the transistor 107 is turned on, the level of the potential 204 ofthe node 117 becomes equivalent to the level of the potential V2 whichis applied through the terminal 104A.

The potential 205 of the node 118 is at V2+Vb by capacitive coupling ofthe capacitor 108. The potential Vb is preferably higher than thethreshold voltage of the transistor 110 and lower than V1-V2.

When the potential 205 of the node 118 is at V2+Vb, the transistor 110is turned on. When the transistor 110 is turned on, the level of thepotential 204 of the node 117 becomes equivalent to the level of thepotential V2 which is applied through the terminal 104C.

When the potential 204 of the node 117 is at V2, the transistor 113 andthe transistor 114 are turned off. Note that the transistor 113 isturned off after the first clock signal 202 which is input through theterminal 103A enters into a low state in many cases. This is because thepotential 204 of the node 117 is often delayed or dulled as compared tothe first clock signal 202. When the transistor 113 is turned off afterthe first clock signal 202 enters into the low state, the node 119enters into a floating state with the potential thereof kept at thepotential V2 which is applied through the terminal 104E.

When the node 119 is in the floating state, the transistor 111 and thetransistor 115 are kept off.

The capacitor 112 holds a potential difference between the potential ofthe first clock signal 202 which is input through the terminal 103A andthe potential 206 of the node 119, i.e., a potential difference betweenthe potential of the first clock signal 202 which is in the low stateand the potential V2 which is applied through the terminal 104E.

In this case, the level of the potential of the output signal 207 whichis output through the terminal 105 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 104G. The aboveis the operation in the third period.

Next, in the fourth period, the first control signal 201 which is in thelow state is input through the terminal 100; the second control signal208 which is in the low state is input through the terminal 101; thesecond clock signal 203 which is in the low state is input through theterminal 102; the first clock signal 202 which is in the high state isinput through the terminal 103. In this case, the transistor 107 and thetransistor 116 are turned off, and the transistor 106 and the transistor109 are kept off.

In this case, the potential 205 of the node 118 is at V2 by thecapacitive coupling of the capacitor 108. Therefore, the transistor 110is turned off.

The potential 206 of the node 119 is at V2+Vc by capacitive coupling ofthe capacitor 112. The potential Vc is preferably higher than thethreshold voltage of the transistor 111 or the threshold voltage of thetransistor 115 and lower than V1−V2.

When the potential 206 of the node 119 is at V2+Vc, the transistor 111and the transistor 115 are turned on.

When the transistor 111 is turned on, the level of the potential 204 ofthe node 117 becomes equivalent to the level of the potential V2 whichis applied through the terminal 104D.

When the potential 204 of the node 117 is at V2, the transistor 113 andthe transistor 114 are turned off.

When the transistor 115 is turned on, the level of the potential of theoutput signal 207 which is output through the terminal 105 becomesequivalent to the level of the potential V2 which is applied through theterminal 104F. The above is the operation in the fourth period.

Next, in the fifth period, the first control signal 201 which is in thelow state is input through the terminal 100; the second control signal208 which is in the low state is input through the terminal 101; thesecond clock signal 203 which is in the high state is input through theterminal 102; the first clock signal 202 which is in the low state isinput through the terminal 103. In this case, the transistor 116 isturned on, and the transistor 106, the transistor 107, and thetransistor 109 are kept off.

In this case, the potential of the node 118 is at V2+Vb by thecapacitive coupling of the capacitor 108. When the potential of the node118 is at V2+Vb, the transistor 110 is turned on. When the transistor110 is turned on, the level of the potential 204 of the node 117 becomesequivalent to the level of the potential V2 which is applied through theterminal 104C.

The potential 206 of the node 119 is at V2 by the capacitive coupling ofthe capacitor 112. When the potential 206 of the node 119 is at V2, thetransistor 111 and the transistor 115 are turned off.

When the potential 204 of the node 117 is at V2, the transistor 113 andthe transistor 114 are turned off.

In this case, the level of the potential of the output signal 207 whichis output through the terminal 105 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 104G. The aboveis the operation in the fifth period.

As described above, in the driver circuit of this embodiment, during thenon-selection period after a reset period (the third period), theoperation in the fourth period and the operation in the fifth period arerepeated plural times. Thus, a potential having a certain level isapplied to the node 117 in any period of the non-selection period, sothat the node 117 can be prevented from entering into a floating state.Therefore, since the adverse effect of noise can be reduced,malfunctions can be suppressed.

In addition, in the operation of the driver circuit of this embodiment,a potential having a certain level can be applied to the node 117 byturning on different transistors in the fourth period and the fifthperiod. Thus, for example, even in the case of using a transistor whicheasily deteriorates, such as a transistor having a semiconductor layerformed using an amorphous semiconductor, deterioration of eachtransistor can be suppressed. Therefore, deviation in timing ofswitching operation of a transistor due to deterioration can be reduced,so that malfunctions can be suppressed.

Here, the circuit simulation results in the case of a conventionaldriver circuit where the potential of the node 117 is controlled byproviding one of the transistor 110 and the transistor 111 in the fourthperiod and the fifth period in FIG. 2 and the circuit simulation resultsin the case of the driver circuit which is an embodiment of the presentinvention, where the potential of the node 117 is controlled byproviding both the transistor 110 and the transistor 111 in the fourthperiod and the fifth period in FIG. 2, are illustrated in FIGS. 25A and25B. Note that the simulation was performed using a SPICE circuitsimulator. In addition, here, as an example, all transistors in aflip-flop circuit were n-channel transistors and V2 was 0 V.

In FIGS. 25A and 25B, FIG. 25A is a graph illustrating changes in thepotential (voltage) of the node 117 in the case where the node 117 iscontrolled using one of the transistor 110 and the transistor 111 in thefourth period and the fifth period; FIG. 25B is a graph illustratingchanges in the potential (voltage) of the node 117 in the case where thenode 117 is controlled using both the transistor 110 and the transistor111 in the fourth period and the fifth period. Note that in FIGS. 25Aand 25B, the unit of voltage is an arbitrary unit (A. U.).

Noise generated in the fourth period and the fifth period after thereset period (the third period) adversely affects the node 117 mainlydue to parasitic capacitance of the transistor 114 illustrated inFIG. 1. First, in a conventional driver circuit, since a transistor iscontrolled using a signal synchronized with one clock signal, after thereset period, the transistor 114 enters into a floating state in thefourth period or the fifth period. When the transistor 114 enters into afloating state, noise is mixed into a normal potential, so that thepotential (voltage) of the node 117 changes by approximately 0.4 A. U.every certain period (the fifth period in FIG. 25A), as illustrated inFIG. 25A.

On the other hand, in the driver circuit which is an embodiment of thepresent invention, the transistor 110 and the transistor 111 arecontrolled using signals synchronized with two clock signals whosephases are opposite to each other. Thus, a predetermined potential isapplied without making the transistors into a floating state in both thefourth period and the fifth period, so that it is apparent that a changein the potential of the node 117 is smaller than or equal to 0.2 A.U.,which is a small change, that is, the adverse effect of noise is little,as illustrated in FIG. 25B. Accordingly, it is apparent that by using aplurality of transistors and turning on the transistor 110 or thetransistor 111 in the fourth period and the fifth period so that apredetermined potential is applied to the node 117, the adverse effectof noise can be reduced.

Further, in this embodiment, the driver circuit which is an embodimentof the present invention can be formed using a structure which isdifferent from the structure in FIG. 1. A different structure of thedriver circuit of this embodiment is described with reference to FIG. 3.FIG. 3 is a circuit diagram illustrating an example of the structure ofthe driver circuit of this embodiment.

In the different structure of the driver circuit of this embodiment,which is illustrated in FIG. 3, in addition to the circuit structureillustrated in FIG. 1, a transistor 120 and a terminal 104H areprovided.

In FIG. 3, portions denoted by the same reference numerals as in FIG. 1are the same portions as in the driver circuit in FIG. 1, so thatdescription thereof is omitted.

A gate terminal of the transistor 120 is electrically connected to theterminal 100. One of a source terminal and a drain terminal of thetransistor 120 is electrically connected to the gate terminal of thetransistor 111. The other of the source terminal and the drain terminalof the transistor 120 is electrically connected to the terminal 104H.

Since a potential which is the same as the potential applied through theterminals 104A to 104G in FIG. 1 is applied through the terminal 104H,the description in FIG. 1 is incorporated. In addition, the terminals104A to 104H can be electrically connected to each other so as to be oneterminal 104.

The transistor 120 has a function of controlling conduction between theterminal 104H and the node 119 in accordance with the signal which isinput through the terminal 100. By bringing the terminal 104H and thenode 119 into conduction, the potential of the node 119 is set to V1 orV2.

Next, the operation of the driver circuit illustrated in FIG. 3 isdescribed. Note that only the operation of the transistor 120 isdescribed as the operation of the driver circuit in FIG. 3, and thedescription in FIG. 1 is incorporated in description of operation whichis the same as the operation of the elements in the driver circuitillustrated in FIG. 1. In addition, here, as an example of the operationof the driver circuit illustrated in FIG. 3, the case where all thetransistors in the flip-flop circuit are n-channel transistors isdescribed.

In a first period, the first control signal 201 which is in the highstate is input through the terminal 100. In this case, the transistor120 is turned on.

When the transistor 120 is turned on, the level of the potential of thenode 119 becomes equivalent to the level of the potential V2 which isapplied through the terminal 104H. Therefore, the transistor 111 and thetransistor 115 are turned off.

After that, in second to fifth periods, the first control signal 201which is in the low state is input through the terminal 100. In thiscase, the transistor 120 is turned off.

As described above, in the driver circuit illustrated in FIG. 3, inaddition to the advantageous effects of the circuit structure in FIG. 1,the potential of the node 119 can be more surely set to the potential V2in the first period by directly inputting the first control signal 201to the transistor 120 in the first period so that the transistor 120 isturned on.

Further, in this embodiment, the driver circuit which is an embodimentof the present invention can be formed using a structure which isdifferent from the structures in FIG. 1 and FIG. 3. A differentstructure of the driver circuit of this embodiment is described withreference to FIG. 4. FIG. 4 is a circuit diagram illustrating an exampleof the circuit structure of the driver circuit of this embodiment.

In the structure of the driver circuit illustrated in FIG. 4, inaddition to the circuit structure illustrated in FIG. 1, a terminal103C, a terminal 104I, a terminal 104J, a terminal 121, a transistor122, a transistor 123, and a transistor 124 are provided.

Note that in FIG. 4, elements denoted by the same reference numerals asin FIG. 1 are the same elements as in the driver circuit in FIG. 1, sothat the description in FIG. 1 is incorporated.

A gate terminal of the transistor 122 is electrically connected to theother of the source terminal and the drain terminal of the transistor106. One of a source terminal and a drain terminal of the transistor 122is electrically connected to the terminal 103C. The other of the sourceterminal and the drain terminal of the transistor 122 is electricallyconnected to the terminal 121.

A gate terminal of the transistor 123 is electrically connected to thegate terminal of the transistor 111. One of a source terminal and adrain terminal of the transistor 123 is electrically connected to theother of the source terminal and the drain terminal of the transistor122. The other of the source terminal and the drain terminal of thetransistor 123 is electrically connected to the terminal 104I.

A gate terminal of the transistor 124 is electrically connected to theterminal 102B. One of a source terminal and a drain terminal of thetransistor 124 is electrically connected to the other of the sourceterminal and the drain terminal of the transistor 122. The other of thesource terminal and the drain terminal of the transistor 124 iselectrically connected to the terminal 104J.

Since a signal which is the same as the signal input through theterminal 103A and the terminal 103B in FIG. 1 is input through theterminal 103C, the description in FIG. 1 is incorporated. In addition,the terminals 103A to 103C can be electrically connected to each otherso as to be one terminal 103.

Since a potential which is the same as the potential applied through theterminals 104A to 104G in FIG. 1 is applied through the terminal 104Iand the terminal 104J, the description in FIG. 1 is incorporated. Inaddition, the terminals 104A to 104E and the terminals 104I and 104J canbe electrically connected to each other so as to be one terminal 104.

Further, the flip-flop circuit outputs signals generated in theflip-flop circuit through the terminal 121.

The transistor 122 has a function of bringing the terminal 103C and theterminal 121 into conduction in accordance with the potential of thenode 117 so that the level of a potential of the signal which is inputthrough the terminal 103C is made equivalent to the level of a potentialof a signal which is output through the terminal 121. In particular, thetransistor 122 has a function of raising the potential of the node 117in accordance with rise in the potential of the signal which is outputthrough the terminal 121 when the signal which is input through theterminal 103C is changed from the low state to the high state in thecase where the potential of the node 117 is V1. That is, the transistor122 performs so-called bootstrap operation. Note that the bootstrapoperation is often performed using parasitic capacitance between thegate terminal of the transistor 122 and the other of the source terminaland the drain terminal of the transistor 122.

The transistor 123 has a function of controlling conduction between theterminal 104I and the terminal 121 in accordance with the potential ofthe node 119. By bringing the terminal 104I and the terminal 121 intoconduction, the potential of the signal which is output through theterminal 121 is set to V1 or V2.

The transistor 124 has a function of controlling conduction between theterminal 104J and the terminal 121 in accordance with the signal whichis input through the terminal 102B. By bringing the terminal 104J andthe terminal 121 into conduction, the potential of the signal which isoutput through the terminal 121 is set to V1 or V2.

Next, the operation of the driver circuit illustrated in FIG. 4 isdescribed with reference to FIG. 5. FIG. 5 is a timing chartillustrating an example of the operation of the driver circuit of thisembodiment. Note that only the operation of the transistor 122, theoperation of the transistor 123, and the operation of the transistor 124are described as the operation of the driver circuit in FIG. 4, and thedescription of the driver circuit in FIG. 1 is incorporated indescription of operation which is the same as the operation of theelements in the driver circuit illustrated in FIG. 1, as appropriate.Note that the case where the first clock signal is input to the terminal103C in FIG. 5 is described. In addition, here, as an example of theoperation of the driver circuit illustrated in FIG. 4, the case whereall the transistors in the flip-flop circuit are n-channel transistorsis described.

In a first period, in addition to the operation of the circuitillustrated in FIG. 1, the first clock signal 202 which is in the lowstate is input through the terminal 103C. In this case, the transistor124 is turned on.

In this case, the potential 204 of the node 117 is at V1-Vth₁₀₆, so thatthe transistor 113 is turned on. When the transistor 113 is turned on,the transistor 123 is turned off.

When the potential 204 of the node 117 is at V1-Vth₁₀₆, the transistor122 is turned on.

In this case, the level of a potential of an output signal 209 which isoutput through the terminal 121 becomes equivalent to the level of thepotential V2 of the first clock signal which is input through theterminal 103C or the level of the potential V2 which is applied throughthe terminal 104J. The above is the operation in the first period.

Next, in a second period, in addition to the operation of the circuitillustrated in FIG. 1, the first clock signal 202 which is in the highstate is input through the terminal 103C. In this case, the transistor124 is turned off.

In this case, the potential 204 of the node 117 is kept at V1-Vth₁₀₆ andthe transistor 113 is kept on. When the transistor 113 is in an onstate, the transistor 123 is kept off.

In addition, in this case, the node 117 is kept in a floating state andthe potential 204 of the node 117 is kept at V1-Vth₁₀₆.

When the potential 204 of the node 117 is kept at V1-Vth₁₀₆ and apotential of the one of the source terminal and the drain terminal ofthe transistor 122 becomes the potential V1 of the first clock signal202, the potential 204 of the node 117 rises by capacitive coupling ofparasitic capacitance between the gate terminal of the transistor 122and the other of the source terminal and the drain terminal of thetransistor 122 in accordance with the potential of the output signal 209by bootstrap. In this case, the potential 204 of the node 117 rises to avalue which is larger than the sum of the potential V1 of the firstclock signal 202 and the threshold voltage of the transistor 114 or thesum of the potential V1 of the first clock signal 202 and the thresholdvoltage of the transistor 122 (also referred to as Vth₁₂₂), i.e., toV1+Vth₁₁₄+Va or V1+Vth₁₂₂+Va (Va is a given positive number).

When the potential 204 of the node 117 is V1+Vth₁₁₄+Va or V1+Vth₁₂₂+Va,the transistor 122 is kept on.

In this case, the level of the potential of the output signal 209 whichis output through the terminal 121 becomes equivalent to the level ofthe potential V1 of the first clock signal 202 which is input throughthe terminal 103C. The above is the operation in the second period.

Next, in a third period, in addition to the operation of the circuitillustrated in FIG. 1, the first clock signal 202 which is in the lowstate is input through the terminal 103C. In this case, the transistor124 is turned on.

In this case, the potential 205 of the node 118 is at V2+Vb, thetransistor 110 is turned on, and the level of the potential 204 of thenode 117 becomes equivalent to the level of the potential V2. When thepotential 204 of the node 117 is at V2, the transistor 122 is turnedoff.

The potential 206 of the node 119 is kept at the level which isequivalent to the potential V2. When the potential 206 of the node 119is V2, the node 119 enters into a floating state. When the node 119 isin the floating state, the transistor 123 is kept off.

In this case, the level of the potential of the output signal 209 whichis output through the terminal 121 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 104J. The aboveis the operation in the third period.

Next, in a fourth period, in addition to the operation of the circuitillustrated in FIG. 1, the first clock signal 202 which is in the highstate is input through the terminal 103C. In this case, the transistor116 is turned off.

In this case, the potential 206 of the node 119 is at V2+Vc. When thepotential 206 of the node 119 is at V2+Vc, the transistor 123 is turnedon.

The potential 204 of the node 117 is at the potential V2 which isapplied through the terminal 104D. When the potential of the node 117 isat V2, the transistor 122 is turned off.

In this case, the level of the potential of the output signal 209 whichis output through the terminal 121 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 104I. The aboveis the operation in the fourth period.

Next, in a fifth period, in addition to the operation of the circuitillustrated in FIG. 1, the first clock signal 202 which is in the lowstate is input through the terminal 103C. In this case, the transistor124 is turned on.

In this case, the potential 205 of the node 118 is at V2+Vb and thetransistor 110 is turned on. When the transistor 110 is turned on, thelevel of the potential 204 of the node 117 becomes equivalent to thelevel of the potential V2 which is applied through the terminal 104C.

When the potential 204 of the node 117 is at V2, the transistor 122 isturned off.

In addition, the potential 206 of the node 119 is at V2 and thetransistor 123 is turned off.

In this case, the level of the potential of the output signal 209 whichis output through the terminal 121 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 104J. The aboveis the operation in the fifth period.

As described above, in the driver circuit illustrated in FIG. 4, inaddition to the advantageous effects of the circuit structure in FIG. 1,by using a plurality of output signals, one of the output signals isoutput to a flip-flop circuit in the next stage, and the other of theoutput signals is output to a gate terminal of a transistor in a pixel.Thus, an output signal with slight deviation can be output to theflip-flop circuit, so that malfunctions can be suppressed.

Further, in this embodiment, the structure in FIG. 3 and the structurein FIG. 4 can be combined with each other. A different structure of thedriver circuit of this embodiment is described with reference to FIG. 6.FIG. 6 is a circuit diagram illustrating the different structure of thedriver circuit of this embodiment.

In the different structure of the driver circuit of this embodiment,which is illustrated in FIG. 6, in addition to the circuit structureillustrated in FIG. 1, a terminal 103D, a terminal 104K, a terminal104L, a terminal 104M, a terminal 125, a transistor 126, a transistor127, a transistor 128, and a transistor 129 are provided.

In FIG. 6, elements denoted by the same reference numerals as in FIG. 1are the same elements as in the driver circuit in FIG. 1, so that thedescription of each element in FIG. 1 is incorporated.

In FIG. 6, the terminal 103D corresponds to the terminal 103C in FIG. 4;the terminal 104K corresponds to the terminal 104H in FIG. 3; theterminal 104L corresponds to the terminal 104I in FIG. 4; the terminal104M corresponds to the terminal 104J in FIG. 4; the terminal 125correspond to the terminal 121 in FIG. 4; the transistor 126 correspondsto the transistor 120 in FIG. 3; the transistor 127 corresponds to thetransistor 122 in FIG. 4; the transistor 128 corresponds to thetransistor 123 in FIG. 4; the transistor 129 corresponds to thetransistor 124 in FIG. 4. The description of each element in FIG. 3 andFIG. 4 is incorporated in description of each element.

Since the operation of the driver circuit in FIG. 6 is combination ofthe operation of the driver circuit in FIG. 3 and the operation of thedriver circuit in FIG. 4, the description of the operation of the drivercircuit in FIG. 3 and the operation of the driver circuit in FIG. 4 isincorporated.

By using the structure illustrated in FIG. 6, advantageous effects whichare the same as the advantageous effects of the driver circuit havingthe structure illustrated in FIG. 3 and the driver circuit having thestructure illustrated in FIG. 4 can be obtained.

Embodiment 2

In this embodiment, a driver circuit having a structure which isdifferent from the structure in Embodiment 1 is described.

A driver circuit in this embodiment includes a shift register includinga plurality of flip-flop circuits.

In addition, an example of the circuit structure of the flip-flopcircuit is described with reference to FIG. 7. FIG. 7 is a circuitdiagram illustrating an example of the circuit structure of theflip-flop circuit in the driver circuit of this embodiment.

The flip-flop circuit illustrated in FIG. 7 includes a terminal 500, aterminal 501, a terminal 502, a terminal 503, a terminal 504, a terminal505, a transistor 506, a transistor 507, a transistor 508, a transistor509, a capacitor 510, a transistor 511, a transistor 512, a transistor513, and a transistor 514.

Note that although a terminal 502A and a terminal 502B are illustratedas the terminal 502 in this embodiment, the structure of the terminal502 is not limited to this. The terminal 502A and the terminal 502B canbe electrically connected to each other so as to be one terminal 502. Inaddition, although a terminal 503A and a terminal 503B are illustratedas the terminal 503 in this embodiment, the structure of the terminal503 is not limited to this. The terminal 503A and the terminal 503B canbe electrically connected to each other so as to be one terminal 503.

Further, although terminals 504A to 504E are illustrated as the terminal504 in this embodiment, the structure of the terminal 504 is not limitedto this. The terminals 504A to 504E can be electrically connected toeach other so as to be one terminal 504.

A gate terminal of the transistor 506 is electrically connected to theterminal 502A. One of a source terminal and a drain terminal of thetransistor 506 is electrically connected to the terminal 500.

A gate terminal of the transistor 507 is electrically connected to theterminal 500. One of a source terminal and a drain terminal of thetransistor 507 is electrically connected to the gate terminal of thetransistor 507. The other of the source terminal and the drain terminalof the transistor 507 is electrically connected to the other of thesource terminal and the drain terminal of the transistor 506. Note thatalthough not illustrated for convenience, by using a structure where thetransistor 507 is not provided in this embodiment, the circuit area canbe made smaller.

A gate terminal of the transistor 508 is electrically connected to theterminal 501. One of a source terminal and a drain terminal of thetransistor 508 is electrically connected to the other of the sourceterminal and the drain terminal of the transistor 507. The other of thesource terminal and the drain terminal of the transistor 508 iselectrically connected to the terminal 504A. Note that although notillustrated for convenience, by using a structure where the transistor508 is not provided in the flip-flop circuit in the driver circuit ofthis embodiment, the circuit area can be made smaller.

The capacitor 510 includes at least two terminals. One of the terminalsof the capacitor 510 is electrically connected to the terminal 503A.

A gate terminal of the transistor 509 is electrically connected to theother of the terminals of the capacitor 510. One of a source terminaland a drain terminal of the transistor 509 is electrically connected tothe other of the source terminal and the drain terminal of thetransistor 506. The other of the source terminal and the drain terminalof the transistor 509 is electrically connected to the terminal 504B.

A gate terminal of the transistor 511 is electrically connected to theother of the source terminal and the drain terminal of the transistor506. One of a source terminal and a drain terminal of the transistor 511is electrically connected to the gate terminal of the transistor 509.The other of the source terminal and the drain terminal of thetransistor 511 is electrically connected to the terminal 504C.

A gate terminal of the transistor 512 is electrically connected to theother of the source terminal and the drain terminal of the transistor506. One of a source terminal and a drain terminal of the transistor 512is electrically connected to the terminal 503B. The other of the sourceterminal and the drain terminal of the transistor 512 is electricallyconnected to the terminal 505. The potential of the other of the sourceterminal and the drain terminal of the transistor 114 is an outputsignal and is output through the terminal 105. Note that although notillustrated for convenience, in the flip-flop circuit in the drivercircuit of this embodiment, a capacitor can be additionally providedbetween the gate terminal of the transistor 512 and the other of thesource terminal and the drain terminal of the transistor 512.

A gate terminal of the transistor 513 is electrically connected to thegate terminal of the transistor 509. One of a source terminal and adrain terminal of the transistor 513 is electrically connected to theother of the source terminal and the drain terminal of the transistor512. The other of the source terminal and the drain terminal of thetransistor 513 is electrically connected to the terminal 504D.

A gate terminal of the transistor 514 is electrically connected to theterminal 502B. One of a source terminal and a drain terminal of thetransistor 514 is electrically connected to the other of the sourceterminal and the drain terminal of the transistor 512. The other of thesource terminal and the drain terminal of the transistor 514 iselectrically connected to the terminal 504E.

Note that a portion where the other of the source terminal and the drainterminal of the transistor 506, the transistor 507, the transistor 508,the transistor 509, the transistor 511, and the transistor 512 areconnected to each other is referred to as a node 515. Further, a portionwhere the one of the terminals of the capacitor 510 is connected to thetransistor 509, the transistor 511, and the transistor 513 is referredto as a node 516.

In the flip-flop circuit, a first control signal is input through theterminal 500, and a second control signal is input through the terminal501. As each of the first control signal and the second control signal,a digital signal having two states of a high state and a low state canbe used. In the case of using the digital signal, the first controlsignal or the second control signal having a predetermined potential isinput as a first potential (also referred to as V1) through the terminal500 or the terminal 501 when the first control signal or the secondcontrol signal, which is input, is in a high state (also referred to asa high level); the first control signal or the second control signalhaving a potential which is lower than the predetermined potential inthe high state is input as a second potential (also referred to as V2)through the terminal 500 or the terminal 501 when the first controlsignal or the second control signal, which is input, is in a low state(also referred to as a low level). The levels of the potentials in thehigh state and the low state can be set as appropriate considering thelevel of the threshold voltage of each transistor, or the like, forexample. For example, the levels of the potentials in the high state andthe low state are preferably set so that a potential difference betweenthe high state and the low state is larger than the absolute value ofthe threshold voltage of each transistor in the flip-flop circuit.

In the flip-flop circuit illustrated in FIG. 7, a clock signal which isin a first phase (also referred to as a first clock signal or a CKsignal) or a clock signal which is in a second phase (also referred toas a second clock signal or a CKB signal) is input through the terminal502 (also referred to as the terminal 502A and the terminal 502B). Eachof the first clock signal and the second clock signal has two potentiallevels of a high state and a low state. A clock signal having the firstpotential (also referred to as V1) is input when each clock signal is ina high state (also referred to as a high level), and a clock signalhaving the second potential (also referred to as V2) is input when eachclock signal is in a low state (also referred to as a low level). Notethat the levels of the potentials of the first clock signal and thesecond clock signal in a high state are preferably equivalent to thelevels of the potentials of the first control signal and the secondcontrol signal in the high state. The levels of the potentials of thefirst clock signal and the second clock signal in a low state arepreferably equivalent to the levels of the potentials of the firstcontrol signal and the second control signal in the low state. Further,the levels of the potentials in the high state and the low state can beset as appropriate considering the level of the threshold voltage ofeach transistor, or the like, for example. For example, the levels ofthe potentials in the high state and the low state are preferably set sothat a potential difference between the high state and the low state islarger than the absolute value of the threshold voltage of eachtransistor in the flip-flop circuit.

The phase of the first clock signal and the phase of the second clocksignal are different from each other. Specifically, the phase of thefirst clock signal and the phase of the second clock signal are oppositeto each other. For example, in a predetermined period, the second clocksignal is in the low state when the first clock signal is in the highstate, and the second clock signal is in the high state when the firstclock signal is in the low state.

In the flip-flop circuit, the first clock signal or the second clocksignal is input through the terminal 503 (also referred to as theterminal 503A and the terminal 503B). Note that the phase of the clocksignal which is input through the terminal 502 and the phase of thesecond clock signal which is input through the terminal 503 are oppositeto each other. For example, the second clock signal is input through theterminal 503 in the case where the first clock signal is input throughthe terminal 502, and the first clock signal is input through theterminal 503 in the case where the second clock signal is input throughthe terminal 502.

A potential having a predetermined level is applied to the flip-flopcircuit through the terminal 504 (also referred to as the terminals 504Ato 504E). In this case, the level of the potential having thepredetermined level can be set to V1 or V2, for example. That is, thelevel of the potential having the predetermined level can be madeequivalent to the level of a potential of a digital signal such as aclock signal or a control signal in a high state or a low state.

The transistor 506 has a function of controlling conduction between theterminal 500 and the node 515 in accordance with a signal which is inputthrough the terminal 502A. By bringing the terminal 500 and the node 515into conduction, the level of a potential of a signal which is inputthrough the terminal 500 is made equivalent to the level of a potentialof the node 515. In addition, the transistor 506 is turned off when thetransistor 509 is in an on state.

The transistor 507 has a function of controlling conduction between theterminal 500 and the node 515 in accordance with the signal which isinput through the terminal 500. By bringing the terminal 500 and thenode 515 into conduction, the potential of the node 515 is set to V1 orV2. After that, the terminal 500 and the node 515 are brought out ofconduction, so that the node 515 enters into a floating state.

The transistor 508 has a function of controlling conduction between theterminal 504A and the node 515 in accordance with a signal which isinput through the terminal 501. By bringing the terminal 504A and thenode 515 into conduction, the potential of the node 515 is set to V1 orV2.

The transistor 509 has a function of controlling conduction between theterminal 504B and the node 515 in accordance with a potential of thenode 516. By bringing the terminal 504B and the node 515 intoconduction, the potential of the node 515 is set to V1 or V2. Inaddition, the transistor 509 has a function of entering into an offstate when the transistor 506 is on.

The capacitor 510 has a function of changing the potential of the node516 by capacitive coupling in accordance with a signal which is inputthrough the terminal 503A. For example, the capacitor 510 sets thepotential of the node 516 to V1 by capacitive coupling in the case wherethe signal which is input through the terminal 503A is changed from alow state to a high state. On the other hand, the capacitor 510 sets thepotential of the node 516 to V1 or V2 by capacitive coupling in the casewhere the signal which is input through the terminal 503A is changedfrom the high state to the low state.

The transistor 511 has a function of controlling conduction between theterminal 504C and the node 516 in accordance with the potential of thenode 515. By bringing the terminal 504C and the node 516 intoconduction, the potential of the node 516 is set to V1 or V2.

The transistor 512 has a function of controlling conduction between theterminal 503B and the terminal 505 in accordance with the potential ofthe node 515. By bringing the terminal 503B and the terminal 505 intoconduction, a potential of a signal which is input through the terminal503B is made equivalent to a potential of a signal which is outputthrough the terminal 505. Further, the transistor 512, for example, isan n-channel transistor and has a function of raising the potential ofthe node 515 in accordance with rise in the potential of the signalwhich is output through the terminal 505 when the signal which is inputthrough the terminal 503B is changed from the low state to the highstate in the case where the potential of the node 515 is V1. That is,the transistor 512 performs so-called bootstrap operation. Note that thebootstrap operation is often performed using parasitic capacitancebetween the gate terminal of the transistor 512 and the other of thesource terminal and the drain terminal of the transistor 512.

The transistor 513 has a function of controlling conduction between theterminal 504D and the terminal 505 in accordance with the potential ofthe node 516. By bringing the terminal 504D and the terminal 505 intoconduction, the potential of the signal which is output through theterminal 505 is set to V1 or V2.

The transistor 514 has a function of controlling conduction between theterminal 504E and the terminal 505 in accordance with a signal which isinput through the terminal 502B. By bringing the terminal 504E and theterminal 505 into conduction, the potential of the signal which isoutput through the terminal 505 is set to V1 or V2.

Note that since all the transistors can have the same conductivity typein the driver circuit of this embodiment, manufacturing steps can besimplified. Therefore, manufacturing cost can be reduced and yield canbe improved. Further, a semiconductor device such as a large displaypanel can be easily manufactured. In the driver circuit of thisembodiment in FIG. 7, all the transistors can be n-channel transistorsor p-channel transistors.

Next, the operation of the driver circuit illustrated in FIG. 7 isdescribed with reference to FIG. 8. FIG. 8 is a timing chartillustrating an example of the operation of the driver circuitillustrated in FIG. 7. Note that here, as an example, the first clocksignal is input through the terminal 503 and the second clock signal isinput through the terminal 502. In addition, here, as an example of theoperation of the driver circuit illustrated in FIG. 7, the case whereall the transistors in the flip-flop circuit are n-channel transistorsis described.

As for the operation of the driver circuit illustrated in FIG. 7,predetermined operation in a certain period is repeated, as illustratedin FIG. 8. The certain period is divided into a selection period and anon-selection period. Further, the selection period and thenon-selection period are divided into a first period, a second period, athird period, a fourth period, and a fifth period. In FIG. 8, the firstperiod, the third period, the fourth period, and the fifth period arethe non-selection period, and the second period is the selection period.

First, in the first period, a first control signal 601 which is in ahigh state is input through the terminal 500; a second control signal607 which is in a low state is input through the terminal 501; a secondclock signal 603 which is in a high state is input through the terminal502; a first clock signal 602 which is in a low state is input throughthe terminal 503. Accordingly, the transistor 506, the transistor 507,and the transistor 514 are turned on, and the transistor 508 is turnedoff.

When the transistor 506 and the transistor 507 are turned on, apotential 604 of the node 515 rises to a value obtained by subtractingthe threshold voltage of the transistor 506 (also referred to as Vth₅₀₆)from the potential V1 of the second clock signal 603 which is inputthrough the terminal 502A, i.e., to V1-Vth₅₀₆, or a value obtained bysubtracting the threshold voltage of the transistor 507 (also referredto as Vth₅₀₇) from the potential V1 of the first control signal 601which is input through the terminal 500, i.e., to V1-Vth₅₀₇. When thepotential of the node 515 rises to V1-Vth₅₀₆ or V1-Vth₅₀₇, thetransistor 507 is turned off. In this case, the level of the thresholdvoltage of the transistor 506 and the level of the threshold voltage ofthe transistor 507 are preferably equivalent to each other. In FIG. 8,as an example, the potential of the node 515 in the second period is atV1-Vth₅₀₇.

When the potential 604 of the node 515 is at V1-Vth₅₀₇, the transistor511 and the transistor 512 are turned on.

When the transistor 511 is turned on, the level of a potential 605 ofthe node 516 becomes equivalent to the level of the potential V2 whichis applied through the terminal 504C. When the potential of the node 516is at V2, the transistor 509 and the transistor 513 are turned off.

In this case, the level of a potential of an output signal 606 which isoutput through the terminal 505 becomes equivalent to the level of thepotential V2 of the first clock signal 602 which is input through theterminal 503B or the level of the potential V2 which is applied throughthe terminal 504E. The above is the operation in the first period.

Next, in the second period, the first control signal 601 which is in alow state is input through the terminal 500; the second control signal607 which is in the low state is input through the terminal 501; thesecond clock signal 603 which is in a low state is input through theterminal 502; the first clock signal 602 which is in a high state isinput through the terminal 503A and the terminal 503B. In this case, thetransistor 506, the transistor 507, and the transistor 514 are turnedoff, and the transistor 508 is kept off.

In this case, the potential 604 of the node 515 is kept at V1-Vth₅₀₇,and the transistor 511 is kept on. In addition, when the potential 604of the node 515 is kept at V1-Vth₅₀₇, the potential 605 of the node 516is kept at the potential V2 which is applied through the terminal 504C,and the transistor 509 and the transistor 513 are kept off.

When the transistor 506, the transistor 507, the transistor 508, thetransistor 509, and the transistor 513 are in an off state as describedabove, the node 515 is kept in the floating state, and the potential 604of the node 515 is kept at V1-Vth₅₀₇.

When the potential 604 of the node 515 is kept at V1-Vth₅₀₇ and apotential of the one of the source terminal and the drain terminal ofthe transistor 512 becomes the potential V1 of the first clock signal602, a potential of the output signal 606 which is output through theterminal 505 rises. Then, since the node 515 is in the floating state,the potential 604 of the node 515 rises by capacitive coupling ofparasitic capacitance between the gate terminal of the transistor 512and the other of the source terminal and the drain terminal of thetransistor 512 in accordance with the potential of the output signal 606by bootstrap.

The potential 604 of the node 515 rises to a value which is larger thanthe sum of the potential V1 of the first clock signal 602 and thethreshold voltage of the transistor 512 (also referred to as Vth₅₁₂),i.e., V1+Vth₅₁₂+Va (Va is a given positive number). In this case, thetransistor 512 is kept on.

In this case, the level of the potential of the output signal 606 whichis output through the terminal 505 becomes equivalent to the level ofthe potential V1 which is applied through the terminal 503B. The aboveis the operation in the second period.

Next, in the third period, the first control signal 601 which is in thelow state is input through the terminal 500; the second control signal607 which is in a high state is input through the terminal 501; thesecond clock signal 603 which is in the high state is input through theterminal 502A and the terminal 502B; the first clock signal 602 which isin the low state is input through the terminal 503A and the terminal503B. In this case, the transistor 506, the transistor 508, and thetransistor 514 are turned on, and the transistor 507 is kept off.

When the transistor 506 and the transistor 508 are turned on, the levelof the potential of the node 515 becomes equivalent to the level of thepotential V2 of the first control signal which is input through theterminal 500 or the level of the potential V2 which is applied throughthe terminal 504A.

When the potential 604 of the node 515 is at V2, the transistor 511 andthe transistor 512 are turned off. Note that the transistor 511 is oftenturned off after the first clock signal 602 which is input through theterminal 502B enters into a low state. This is because the potential 604of the node 515 is often delayed or dulled as compared to the firstclock signal 602. When the transistor 511 is turned off after the firstclock signal 602 enters into the low state, the node 516 enters into afloating state with the potential thereof kept at the potential V2 whichis applied through the terminal 504C.

When the node 516 is in the floating state, the transistor 509 and thetransistor 513 are kept off.

The capacitor 510 holds a potential difference between a potential ofthe first clock signal 602 which is input through the terminal 503A andthe potential of the node 516, i.e., a potential difference between thepotential of the first clock signal 602 which is in the low state andthe potential V2 which is applied through the terminal 504C.

In this case, the level of the potential of the output signal 606 whichis output through the terminal 505 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 504E. The aboveis the operation in the third period.

Next, in the fourth period, the first control signal 601 which is in thelow state is input through the terminal 500; the second control signal607 which is in the low state is input through the terminal 501; thesecond clock signal 603 which is in the low state is input through theterminal 502A and the terminal 502B; the first clock signal 602 which isin the high state is input through the terminal 503A and the terminal503B. In this case, the transistor 506, the transistor 508, and thetransistor 514 are turned off, and the transistor 507 is kept off.

The potential 605 of the node 516 is at V2+Vb by capacitive coupling ofthe capacitor 510. The potential Vb is preferably higher than thethreshold voltage of the transistor 509 or the threshold voltage of thetransistor 513, and lower than V1-V2.

When the potential 605 of the node 516 is at V2+Vc, the transistor 509and the transistor 513 are turned on. When the transistor 509 and thetransistor 513 are turned on, the level of the potential 604 of the node515 becomes equivalent to the level of the potential V2 which is appliedthrough the terminal 504B or the level of the potential V2 which isapplied through the terminal 504D.

When the potential 604 of the node 515 is at V2, the transistor 511 andthe transistor 512 are turned off.

In this case, the level of the potential of the output signal 606 whichis output through the terminal 505 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 504D. The aboveis the operation in the fourth period.

Next, in the fifth period, the first control signal 601 which is in thelow state is input through the terminal 500; the second control signal607 which is in the low state is input through the terminal 501; thesecond clock signal 603 which is in the high state is input through theterminal 502A and the terminal 502B; the first clock signal 602 which isin the high state is input through the terminal 503A and the terminal503B. In this case, the transistor 506 and the transistor 514 are turnedon, and the transistor 507 and the transistor 508 are kept off.

In this case, the potential 605 of the node 516 is at V2 by thecapacitive coupling of the capacitor 510. When the potential 605 of thenode 516 is at V2, the transistor 509 and the transistor 513 are turnedoff.

When the potential 604 of the node 515 is at V2, the transistor 511 andthe transistor 512 are turned off.

In this case, the level of the potential of the output signal 606 whichis output through the terminal 505 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 504D. The aboveis the operation in the fifth period.

Note that in the operation of the driver circuit of this embodiment,during the non-selection period after the third period, the operation inthe fourth period and the operation in the fifth period are repeatedplural times. Thus, a potential having a certain level is applied to thenode 515 in any period of the non-selection period, so that the node 515can be prevented from entering into a floating state. Therefore, sincethe adverse effect of noise can be reduced, malfunctions can besuppressed.

In addition, in the operation of the driver circuit of this embodiment,a potential having a certain level can be applied to the node 515 byturning on different transistors (in this embodiment, the transistor 506and the transistor 509) in the fourth period and the fifth period. Thus,for example, even in the case of using a transistor which has asemiconductor layer formed using an amorphous semiconductor,deterioration of each transistor can be suppressed. Therefore, deviationin timing of switching operation of a transistor due to deteriorationcan be reduced, so that malfunctions can be suppressed.

Since the number of elements included in the diver circuit in thisembodiment can be made smaller than the number of elements included inthe driver circuit of the above embodiment, the circuit area can be madesmaller.

Further, in this embodiment, a driver circuit which is an embodiment ofthe present invention can be formed using a structure which is differentfrom the structure in FIG. 7. A different structure of the drivercircuit of this embodiment is described with reference to FIG. 9. FIG. 9is a circuit diagram illustrating an example of the structure of thedriver circuit of this embodiment.

In the driver circuit illustrated in FIG. 9, in addition to the circuitstructure illustrated in FIG. 7, a terminal 504F and a transistor 517are provided.

Note that in the driver circuit illustrated in FIG. 9, portions denotedby the same reference numerals as in FIG. 7 are the same portions as inthe driver circuit in FIG. 7, so that description thereof is omitted.

A gate terminal of the transistor 517 is electrically connected to theterminal 500. One of a source terminal and a drain terminal of thetransistor 517 is electrically connected to the gate terminal of thetransistor 509. The other of the source terminal and the drain terminalof the transistor 517 is electrically connected to the terminal 504F.

In the driver circuit illustrated in FIG. 9, a potential which isequivalent to the potential applied through the terminals 504A to 504Ein FIG. 7 is applied through the terminal 504F. In addition, theterminals 504A to 504F can be electrically connected to each other so asto be one terminal 504.

The transistor 517 has a function of controlling conduction between theterminal 504F and the node 516 in accordance with the signal which isinput through the terminal 500. By bringing the terminal 504F and thenode 516 into conduction, the potential of the node 516 is set to V1 orV2.

Next, the operation of the driver circuit illustrated in FIG. 9 isdescribed. Note that only the operation of the transistor 517 isdescribed as the operation of the driver circuit in FIG. 9, and theoperation of elements except for the transistor 517 is the same as theoperation of the driver circuit illustrated in FIG. 7; therefore,description thereof is omitted. In addition, here, as an example of theoperation of the driver circuit illustrated in FIG. 9, the case whereall the transistors in the flip-flop circuit are n-channel transistorsis described.

In a first period, the first control signal 601 which is in the highstate is input through the terminal 500. In this case, the transistor517 is turned on.

When the transistor 517 is turned on, the level of the potential of thenode 516 becomes equivalent to the level of the potential V2 which isapplied through the terminal 504F.

After that, in second to fifth periods, the first control signal 601which is in the low state is input through the terminal 500, so that thetransistor 517 is turned off.

As described above, in the driver circuit illustrated in FIG. 9, inaddition to the advantageous effects of the circuit structure in FIG. 7,the potential of the node 516 can be more surely set to the potential V2in the first period by directly inputting the first control signal 601to the transistor 517 in the first period so that the transistor 517 isturned on.

Further, in this embodiment, a driver circuit which is an embodiment ofthe present invention can be formed using a structure which is differentfrom the structures in FIG. 7 and FIG. 9. A different structure of thedriver circuit of this embodiment is described with reference to FIG.10. FIG. 10 is a circuit diagram illustrating an example of the circuitstructure of the driver circuit of this embodiment.

In the structure of the driver circuit illustrated in FIG. 10, inaddition to the circuit structure illustrated in FIG. 7, a terminal503C, a terminal 504G a terminal 504H, a terminal 518, a transistor 519,a transistor 520, and a transistor 521 are provided.

Note that in FIG. 10, elements denoted by the same reference numerals asin FIG. 7 are the same elements as in the driver circuit in FIG. 7, sothat the description of each element in FIG. 7 is incorporated.

A gate terminal of the transistor 519 is electrically connected to theother of the source terminal and the drain terminal of the transistor506. One of a source terminal and a drain terminal of the transistor 519is electrically connected to the terminal 503C.

A gate terminal of the transistor 520 is electrically connected to thegate terminal of the transistor 509. One of a source terminal and adrain terminal of the transistor 520 is electrically connected to theother of the source terminal and the drain terminal of the transistor519. The other of the source terminal and the drain terminal of thetransistor 520 is electrically connected to the terminal 504G.

A gate terminal of the transistor 521 is electrically connected to thegate terminal of the transistor 514. One of a source terminal and adrain terminal of the transistor 521 is electrically connected to theother of the source terminal and the drain terminal of the transistor519. The other of the source terminal and the drain terminal of thetransistor 521 is electrically connected to the terminal 504H.

The transistor 519 has a function of bringing the terminal 503C and theterminal 518 into conduction in accordance with the potential of thenode 515 so that a potential of a signal which is input through theterminal 503C is made equivalent to a potential of a signal which isoutput through the terminal 518. In particular, the transistor 519 has afunction of raising the potential of the node 515 in accordance withrise in a potential of the other of the source terminal and the drainterminal of the transistor 519 when the signal which is input throughthe terminal 503C is changed from the low state to the high state in thecase where the potential of the node 515 is V1. That is, the transistor519 performs so-called bootstrap operation. The bootstrap operation isoften performed using parasitic capacitance between the gate terminal ofthe transistor 519 and the other of the source terminal and the drainterminal of the transistor 519.

The transistor 520 has a function of bringing the terminal 504G and theterminal 518 into conduction in accordance with the potential of thenode 516 so that the potential of the signal which is output through theterminal 518 is set to V1 or V2.

The transistor 521 has a function of bringing the terminal 504H and theterminal 518 into conduction in accordance with the signal which isinput through the terminal 502B so that the potential of the signalwhich is output through the terminal 518 is set to V1 or V2.

Next, the operation of the driver circuit illustrated in FIG. 10 isdescribed with reference to FIG. 11. FIG. 11 is a timing chartillustrating an example of the operation of the driver circuit of thisembodiment. Note that only the operation of the transistor 519, theoperation of the transistor 520, and the operation of the transistor 521are described as the operation of the driver circuit in FIG. 10, and thedescription of the driver circuit in FIG. 7 is incorporated indescription of operation which is the same as the operation of theelements in the driver circuit illustrated in FIG. 7, as appropriate.Note that the case where the first clock signal is input to the terminal503C in FIG. 10 is described. Here, as an example of the operation ofthe driver circuit illustrated in FIG. 10, the case where all thetransistors in the flip-flop circuit are n-channel transistors isdescribed.

In a first period, in addition to the operation of the circuitillustrated in FIG. 7, the first clock signal 602 which is in the lowstate is input through the terminal 503C. In this case, the transistor521 is turned on.

In this case, the potential 604 of the node 515 is at V1-Vth₅₀₇, so thatthe transistor 511 is turned on. When the transistor 511 is turned on,the transistor 520 is turned off.

When the potential 604 of the node 515 is at V1-Vth₅₀₇, the transistor512 is turned on.

In this case, the level of the potential of an output signal 608 whichis output through the terminal 518 becomes equivalent to the level ofthe potential V2 of the first clock signal which is input through theterminal 503C or the level of the potential V2 which is applied throughthe terminal 504H. The above is the operation in the first period.

Next, in a second period, in addition to the operation of the circuitillustrated in FIG. 7, the first clock signal 602 which is in the highstate is input through the terminal 503C. In this case, the transistor521 is turned off.

In this case, the potential 604 of the node 515 is kept at V1-Vth₅₀₇ andthe transistor 511 is kept on. When the transistor 511 is kept on, thetransistor 520 is kept off.

In addition, in this case, the node 515 is kept in a floating state andthe potential 604 of the node 515 is kept at V1-Vth₅₀₇.

When the potential 604 of the node 515 is kept at V1-Vth₅₀₇ and thepotential of the one of the source terminal and the drain terminal ofthe transistor 519 becomes the potential V1 of the first clock signal602, the potential 604 of the node 515 rises by capacitive coupling ofparasitic capacitance between the gate terminal of the transistor 519and the other of the source terminal and the drain terminal of thetransistor 519 in accordance with the potential of the output signal608. In this case, the potential 604 of the node 515 rises to a valuewhich is larger than the sum of the potential V1 of the first clocksignal 602 and the threshold voltage of the transistor 512 (alsoreferred to as Vth₅₁₂) or the sum of the potential V1 of the first clocksignal 602 and the threshold voltage of the transistor 519 (alsoreferred to as Vth₅₁₉), i.e., to V1+Vth₅₁₂+Va or V1+Vth₅₁₉+Va (Va is agiven positive number).

When the potential 604 of the node 515 is V1+Vth₅₁₂+Va or V1+Vth₅₁₉+Va,the transistor 519 is kept on.

In this case, the level of the potential of the output signal 608 whichis output through the terminal 518 becomes equivalent to the level ofthe potential V1 of the first clock signal 602 which is input throughthe terminal 503C. The above is the operation in the second period.

Next, in a third period, in addition to the operation of the circuitillustrated in FIG. 7, the first clock signal 602 which is in the lowstate is input through the terminal 503C. In this case, the transistor521 is turned on.

In this case, the potential 605 of the node 516 is kept at the levelwhich is equivalent to the potential V2. When the potential 605 of thenode 516 is V2, the node 516 enters into a floating state. When the node516 is in the floating state, the transistor 520 is kept off.

In this case, the level of the potential of the output signal 608 whichis output through the terminal 518 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 504H. The aboveis the operation in the third period.

Next, in a fourth period, in addition to the operation of the circuitillustrated in FIG. 7, the first clock signal 602 which is in the highstate is input through the terminal 503C. In this case, the transistor521 is turned off.

In this case, the potential 605 of the node 516 is at V2+Vb. When thepotential 605 of the node 516 is at V2+Vb, the transistor 520 is turnedon.

The level of the potential 604 of the node 515 becomes equivalent to thelevel of the potential V2 which is applied through the terminal 504B.When the potential 604 of the node 515 is at V2, the transistor 519 isturned off.

In this case, the level of the potential of the output signal 608 whichis output through the terminal 518 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 504G. The aboveis the operation in the fourth period.

Next, in a fifth period, in addition to the operation of the circuitillustrated in FIG. 7, the first clock signal 602 which is in the lowstate is input through the terminal 503C. In this case, the transistor521 is turned on.

When the potential 604 of the node 515 is at V2, the transistor 519 isturned off.

When the potential 605 of the node 516 is at V2, the transistor 520 isturned off.

In this case, the level of the potential of the output signal 608 whichis output through the terminal 518 becomes equivalent to the level ofthe potential V2 which is applied through the terminal 504H. The aboveis the operation in the fifth period.

As described above, in the flip-flop circuit in the driver circuitillustrated in FIG. 10, by using a plurality of output signals, one ofthe output signals is output to a flip-flop circuit in the next stage,and the other of the output signals is output to a gate terminal of atransistor in a pixel. Thus, an output signal with slight deviation canbe output to the flip-flop circuit, so that malfunctions can besuppressed.

Further, as the flip-flop circuit in the driver circuit of thisembodiment, the structure in FIG. 7 and the structure in FIG. 10 can becombined with each other. A different structure of the flip-flop circuitin the driver circuit of this embodiment is described with reference toFIG. 12. FIG. 12 is a circuit diagram illustrating the differentstructure of the flip-flop circuit in the driver circuit of thisembodiment.

In the different structure of the flip-flop circuit in the drivercircuit of this embodiment, which is illustrated in FIG. 12, in additionto the circuit structure illustrated in FIG. 7, a terminal 503D, aterminal 504I, a terminal 504J, a terminal 504K, a terminal 522, atransistor 523, a transistor 524, a transistor 525, and a transistor 526are provided.

In FIG. 12, elements denoted by the same reference numerals as in FIG. 7are the same elements as in the driver circuit in FIG. 7, so that thedescription of each element in FIG. 7 is incorporated as appropriate.

In FIG. 12, the terminal 503D corresponds to the terminal 503C in FIG.10; the terminal 504I corresponds to the terminal 504F in FIG. 9; theterminal 504J corresponds to the terminal 504G in FIG. 10; the terminal504K corresponds to the terminal 504H in FIG. 10; the terminal 522corresponds to the terminal 518 in FIG. 10; the transistor 523corresponds to the transistor 517 in FIG. 9; the transistor 524corresponds to the transistor 519 in FIG. 10; the transistor 525corresponds to the transistor 520 in FIG. 10; the transistor 526corresponds to the transistor 521 in FIG. 10. The description of eachelement in FIG. 9 and FIG. 10 is incorporated in description of eachelement as appropriate.

Since the operation of the driver circuit in FIG. 12 is combination ofthe operation of the driver circuit in FIG. 9 and the operation of thedriver circuit in FIG. 10, the description of the operation of thedriver circuit in FIG. 9 and the operation of the driver circuit in FIG.10 is incorporated as appropriate.

By using the structure illustrated in FIG. 12 as described above, theadvantageous effects of the driver circuits illustrated in FIG. 9 andFIG. 10 can be obtained.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, the structure of a display device including thedriver circuit which is an embodiment of the present invention isdescribed.

First, the structure of a display device of this embodiment is describedwith reference to FIG. 13. FIG. 13 is a block diagram illustrating anexample of the structure of the display device of this embodiment.

The display device illustrated in FIG. 13 includes a pixel portion 700,a signal line driver circuit 701, a scan line driver circuit 702, acontrol circuit 703, a clock signal generation circuit 704, a signalline 705A, a signal line 705B, a scan line 706A, a scan line 706B, ascan line 706C, a scan line 706D, a clock signal line 707, and a clocksignal line 708. Note that in the display device illustrated in FIG. 13,the scan line 706A, the scan line 706B, the scan line 706C, or the scanline 706D is simply referred to as a scan line 706. Note that in thedisplay device illustrated in FIG. 13, the signal line 705A or thesignal line 705B is simply referred to as a signal line 705. Inaddition, although two signal lines and four scan lines are illustratedin FIG. 13, the number of signal lines and the number of scan lines arenot particularly limited in the display device of this embodiment. Thenumber of signal lines and the number of scan lines can be differentfrom the above. By increasing the number of signal lines and the numberof scan lines, images can be displayed even in the case of increasingthe number of pixels.

The pixel portion 700 includes a plurality of pixels 709. Note thatalthough only eight pixels 709 are illustrated in FIG. 13, the number ofthe pixels 709 is not limited to this. In the display device of thisembodiment, the number of the pixels 709 can be different from theabove. For example, if the pixel portion has the same size, images canbe displayed clearly by increasing the number of pixels.

The pixel 709 in the pixel portion 700 is electrically connected to thesignal line driver circuit 701 through any one of the plurality ofsignal lines 705 and is electrically connected to the scan line drivercircuit 702 through any one of the plurality of scan lines 706.

The scan line driver circuit 702 includes a shift register. The shiftregister includes a flip-flop circuit 710A which is a first flip-flopcircuit (also referred to as a flip-flop circuit in a first stage), aflip-flop circuit 710B which is a second flip-flop circuit (alsoreferred to as a flip-flop circuit in a second stage), a flip-flopcircuit 710C which is a third flip-flop circuit (also referred to as aflip-flop circuit in a third stage), and a flip-flop circuit 710D whichis a fourth flip-flop circuit (also referred to as a flip-flop circuitin a fourth stage). Note that the flip-flop circuit 710A, the flip-flopcircuit 710B, the flip-flop circuit 710C, or the flip-flop circuit 710Dis simply referred to as a flip-flop circuit 710. Note that in thedisplay device of this embodiment, the number of flip-flop circuits isnot limited to the number of the flip-flop circuits illustrated in FIG.13. The number of the flip-flop circuits can be different from the above((N pieces of stages) (N is a natural number)). For example, it iseffective to increase the number of the flip-flop circuits in the caseof increasing the area of the pixel portion because more signal linescan be controlled.

In the display device of this embodiment, the structure of any one ofthe flip-flop circuits in Embodiments 1 to 3 can be used for theflip-flop circuit 710. The case where the structure of the flip-flop inFIG. 1 is used in the display device illustrated in FIG. 13 is describedas an example. Note that although an example in which the driver circuitwhich is an embodiment of the present invention is used as the scan linedriver circuit in the display device illustrated in FIG. 13 isdescribed, the example of the display device is not limited to this. Inthe display device of this embodiment, the driver circuit which is anembodiment of the present invention can also be applied to the signalline driver circuit.

For example, in the case of using a structure where the flip-flopcircuit 710 has N pieces of stages (N is a natural number of 2 or more),in a flip-flop circuit in a first stage, the terminal 100 illustrated inFIG. 1 is electrically connected to the control circuit 703, and theterminal 105 illustrated in FIG. 1 is electrically connected to thepixel 709 through the first scan line 706.

In the flip-flop circuit 710 in an N^(th) stage, the terminal 100illustrated in FIG. 1 is electrically connected to the terminal 105 inthe flip-flop circuit 710 in an (N−1)^(th) stage, and the terminal 105illustrated in FIG. 1 is electrically connected to the terminal 101illustrated in FIG. 1 in the flip-flop circuit 710 in the (N−1)^(th)stage and is electrically connected to the pixel 709 through a K^(th)scan line 706.

In the flip-flop circuit 710 in an odd-numbered stage, the terminal 102illustrated in FIG. 1 is electrically connected to the clock signalgeneration circuit 704 through the clock signal line 708, and theterminal 103 illustrated in FIG. 1 is electrically connected to theclock signal generation circuit 704 through the clock signal line 707.

In the flip-flop circuit 710 in an even-numbered stage, the terminal 102illustrated in FIG. 1 is electrically connected to the clock signalgeneration circuit 704 through the clock signal line 707, and theterminal 103 illustrated in FIG. 1 is electrically connected to theclock signal generation circuit 704 through the clock signal line 708.

In addition, the structure of the scan line driver circuit 702illustrated in FIG. 13 is specifically described.

In the scan line driver circuit 702 illustrated in FIG. 13, in theflip-flop circuit 710A, the terminal 100 illustrated in FIG. 1 iselectrically connected to the control circuit 703; the terminal 102illustrated in FIG. 1 is electrically connected to the clock signalgeneration circuit 704 through the clock signal line 708; the terminal103 illustrated in FIG. 1 is electrically connected to the clock signalgeneration circuit 704 through the clock signal line 707; the terminal105 illustrated in FIG. 1 is electrically connected to the pixel 709through the scan line 706A.

In the flip-flop circuit 710B, the terminal 100 illustrated in FIG. 1 iselectrically connected to the terminal 105 in the flip-flop circuit710A; the terminal 102 illustrated in FIG. 1 is electrically connectedto the clock signal generation circuit 704 through the clock signal line707; the terminal 103 illustrated in FIG. 1 is electrically connected tothe clock signal generation circuit 704 through the clock signal line708; the terminal 105 illustrated in FIG. 1 is electrically connected tothe terminal 101 illustrated in FIG. 1 in the flip-flop circuit 710A andis electrically connected to the pixel 709 through the scan line 706B.

In the flip-flop circuit 710C, the terminal 100 illustrated in FIG. 1 iselectrically connected to the terminal 105 in the flip-flop circuit710B; the terminal 102 illustrated in FIG. 1 is electrically connectedto the clock signal generation circuit 704 through the clock signal line708; the terminal 103 illustrated in FIG. 1 is electrically connected tothe clock signal generation circuit 704 through the clock signal line707; the terminal 105 illustrated in FIG. 1 is electrically connected tothe terminal 101 illustrated in FIG. 1 in the flip-flop circuit 710B andis electrically connected to the pixel 709 through the scan line 706C.

In the flip-flop circuit 710D, the terminal 100 illustrated in FIG. 1 iselectrically connected to the terminal 105 in the flip-flop circuit710C; the terminal 102 illustrated in FIG. 1 is electrically connectedto the clock signal generation circuit 704 through the clock signal line707; the terminal 103 illustrated in FIG. 1 is electrically connected tothe clock signal generation circuit 704 through the clock signal line708; the terminal 105 illustrated in FIG. 1 is electrically connected tothe terminal 101 illustrated in FIG. 1 in the flip-flop circuit 710C andis electrically connected to the pixel 709 through the scan line 706D.

The clock signal generation circuit 704 outputs a first clock signalthrough the clock signal line 707 and outputs a second clock signalthrough the clock signal line 708. Note that since the first clocksignal and the second clock signal are the same as the first clocksignal and the second clock signal in Embodiment 1, the description inEmbodiment 1 is incorporated as appropriate.

From the control circuit 703, a start signal is output as a firstcontrol signal for starting the operation of the flip-flop circuit. Notethat since the start signal is the same as the first control signal inEmbodiment 1, the description of the first control signal in Embodiment1 is incorporated. In addition, a structure where the control circuit703 is electrically connected to the signal line driver circuit 701 canbe used. By using the structure where the control circuit 703 and thesignal line driver circuit 701 are electrically connected to each other,desired operation can be performed using a control signal also in thesignal line driver circuit 701.

Next, the operation of the display device illustrated in FIG. 13 isdescribed.

First, the operation of the scan line driver circuit 702 is describedwith reference to FIG. 14. FIG. 14 is a timing chart illustrating anexample of the operation of the scan line driver circuit in the displaydevice illustrated in FIG. 13. Here, as an example, the case where theflip-flop circuit is formed using an n-channel transistor is described.

The operation of the scan line driver circuit 702 illustrated in FIG. 13is divided into T (T is a natural number) periods in accordance with thenumber of stages (N) of the flip-flop circuits. Here, as an example, theoperation of the four flip-flop circuits 710A to 710D which areillustrated in FIG. 13 is described assuming that T is 8.

First, in a first period, a start signal 801 which is in a high state isinput to the flip-flop circuit 710A from the control circuit 703 throughthe terminal 100 in the flip-flop circuit 710A; a second clock signal803 which is in a high state is input through the terminal 102; a firstclock signal 802 which is in a low state is input through the terminal103. The operation in the first period here corresponds to the operationin the first period of the timing chart illustrated in FIG. 2 inEmbodiment 1.

Next, in a second period, the start signal 801 which is in a low stateis input to the flip-flop circuit 710A from the control circuit 703through the terminal 100 in the flip-flop circuit 710A; the second clocksignal 803 which is in a low state is input through the terminal 102;the first clock signal 802 which is in a high state is input through theterminal 103. In this case, an output signal 804 which is in a highstate is output to the terminal 100 in the flip-flop circuit 710B andthe scan line 706A through the terminal 105.

In addition, in the second period, the output signal 804 of theflip-flop circuit 710A is input to the flip-flop circuit 710B throughthe terminal 100; the first clock signal 802 which is in the high stateis input through the terminal 102; the second clock signal 803 which isin the low state is input through the terminal 103.

Next, in a third period, the output signal 804 which is in a low stateis input to the flip-flop circuit 710B through the terminal 100; thefirst clock signal 802 which is in the low state is input through theterminal 102; the second clock signal 803 which is in the high state isinput through the terminal 103. In this case, an output signal 805 whichis in a high state is output to the terminal 100 in the flip-flopcircuit 710C, the terminal 101 in the flip-flop circuit 710A, and thescan line 706B through the terminal 105.

In addition, in the third period, the output signal 805 which is in thehigh state is input to the flip-flop circuit 710C through the terminal100; the second clock signal 803 which is in the high state is inputthrough the terminal 102; the first clock signal 802 which is in the lowstate is input through the terminal 103.

Next, in a fourth period, an output signal 805 which is in a low stateis input to the flip-flop circuit 710C through the terminal 100; thesecond clock signal 803 which is in the low state is input through theterminal 102; the first clock signal 802 which is in the high state isinput through the terminal 103. In this case, the output signal 806 isoutput to the terminal 100 in the flip-flop circuit 710D, the terminal101 in the flip-flop circuit 710B, and the scan line 706C through theterminal 105.

In addition, in the fourth period, the output signal 806 which is in thehigh state is input to the flip-flop circuit 710D through the terminal100 as the first control signal; the first clock signal 802 which is inthe high state is input through the terminal 102; the second clocksignal 803 which is in the low state is input through the terminal 103.

Next, in a fifth period, the output signal 806 which is in the low stateis input to the flip-flop circuit 710D through the terminal 100 as thefirst control signal; the first clock signal 802 which is in the lowstate is input through the terminal 102; the second clock signal 803which is in the high state is input through the terminal 103. In thiscase, an output signal 807 is output to the terminal 100 in a flip-flopcircuit in the next stage, the terminal 101 in the flip-flop circuit710C, and the scan line 706D through the terminal 105. The above is theoperation of the scan line driver circuit.

Next, the operation in the pixel portion is described.

First, any one of the plurality of scan lines 706 is selected by thescan line driver circuit 702. A signal is input to the pixel 709 whichis electrically connected to the selected scan line 706 from the signalline driver circuit 701 through the signal line 705, and a predeterminedpotential is applied to a display element so that an image is displayed.Further, images are displayed in different pixels in a similar mannerwhen the other scan lines 706 are sequentially selected. The above isthe operation in the pixel portion.

As described above, by using the driver circuit which is an embodimentof the present invention as the scan line driver circuit in the displaydevice of this embodiment, changes in the amount of signals after theflip-flop circuit is reset can be suppressed. Therefore, malfunctionscan be suppressed. Further, since each scan line can be held at adesired potential, reliability can be improved.

As the display device of this embodiment, a liquid crystal displaydevice can be used, for example. The case where a liquid crystal displaydevice is used is described below.

As the operation mode of a liquid crystal element which can be used in aliquid crystal display device of this embodiment, a TN (twisted nematic)mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching)mode, an MVA (multi-domain vertical alignment), a PVA (patternedvertical alignment) mode, an ASM (axially symmetric aligned micro-cell)mode, an OCB (optical compensated birefringence) mode, an FLC(ferroelectric liquid crystal) mode, an AFLC (antiferroelectric liquidcrystal) mode, or the like can be used.

Next, the structure and operation of a pixel which can be used in theliquid crystal display device of this embodiment are described.

First, the structure of a pixel which can be used in the liquid crystaldisplay device of this embodiment is described with reference to FIG.15A. FIG. 15A is a circuit diagram illustrating an example of thestructure of the pixel portion of the liquid crystal display device ofthis embodiment.

The pixel portion illustrated in FIG. 15A includes a pixel 750, a wiring754, a wiring 755, a wiring 756, and a wiring 757. The pixel 750includes a transistor 751, a liquid crystal element 752, and a capacitor753.

A gate terminal of the transistor 751 is electrically connected to thewiring 755. One of a source terminal and a drain terminal of thetransistor 751 is electrically connected to the wiring 754.

The liquid crystal element 752 includes a first terminal, a secondterminal, and a liquid crystal layer. The first terminal of the liquidcrystal element 752 is electrically connected to the other of the sourceterminal and the drain terminal of the transistor 751. The secondterminal of the liquid crystal element 752 is electrically connected tothe wiring 757.

The capacitor 753 includes at least two terminals. One of the terminalsof the capacitor 753 is electrically connected to the first terminal ofthe liquid crystal element 752. The other of the terminals of thecapacitor 753 is electrically connected to the wiring 756.

The wiring 754 can serve as a signal line, for example. The signal lineis a wiring for sending a data signal, which is input from the outsideof the pixel and has a predetermined potential, to the pixel 750.

The wiring 755 can serve as a scan line. The scan line is a wiring forcontrolling an on state and an off state of the transistor 751.

The wiring 756 can serve as a capacitor line. The capacitor line is awiring for applying predetermined voltage to the one of the terminals ofthe capacitor 753.

The transistor 751 can serve as a switch.

The capacitor 753 can serve as a storage capacitor. The capacitor 753 isa capacitor for holding voltage applied to the liquid crystal element752 for a certain period when the transistor 751 is in an off state.

The wiring 757 can serve as a counter electrode of the liquid crystalelement 752. The counter electrode is a wiring for applyingpredetermined voltage to the liquid crystal element 752.

Note that the function of each wiring is not limited to this, and avariety of functions can be provided. For example, by changing apotential applied to the wiring which serves as the capacitor line, thelevel of voltage applied to the liquid crystal element 752 can becontrolled.

Since the transistor 751 only has to serve as a switch, the transistor751 may be either a p-channel transistor or an n-channel transistor.

Further, a different structure of a pixel which can be used in theliquid crystal display device of this embodiment is described withreference to FIG. 15B. FIG. 15B is a circuit diagram illustrating anexample of the different structure of the pixel portion of the liquidcrystal display device of this embodiment.

The structure of the pixel portion illustrated in FIG. 15B is similar tothe structure of the pixel portion illustrated in FIG. 15A except thatthe wiring 757 is eliminated and the terminal of the liquid crystalelement 752 and the terminal of the capacitor 753 are electricallyconnected to each other. It is particularly preferable to use the pixelportion illustrated in FIG. 15B in the case where the operation mode ofa liquid crystal element is a horizontal electric field mode (e.g., anIPS mode or an FFS mode). This is because electrodes of the liquidcrystal element 752, which are part of the terminals of the liquidcrystal element 752, and electrodes of the capacitor 753 which are partof the terminals of the capacitor 753, can be formed over the samesubstrate in the case where the operation mode of the liquid crystalelement is a horizontal electric field mode, so that the electrodes ofthe liquid crystal element 752 and the electrode of the capacitor 753can be electrically connected to each other easily.

Further, by using the structure of the pixel portion illustrated in FIG.15B, the wiring 757 can be eliminated. Thus, manufacturing steps can besimplified, so that manufacturing cost can be reduced.

Note that in the pixel portion illustrated in FIG. 15A or FIG. 15B, aplurality of pixels can be arranged in matrix. Thus, a display portionof the liquid crystal display device is formed, so that a variety ofimages can be displayed.

The structure of the pixel portion where a plurality of pixels arearranged is described with reference to FIG. 15C. FIG. 15C is a circuitdiagram illustrating an example of the structure of the pixel portion ofthe liquid crystal display device of this embodiment.

In the pixel portion illustrated in FIG. 15C, the plurality of pixels750, one of which is illustrated in FIG. 15A, are arranged in matrix. InFIG. 15C, four pixels are picked up from the plurality of pixels in thepixel portion, and a pixel arranged in an i^(th) column and j^(th) row(i and j are natural numbers) is referred to as a pixel 750 _(—) i, j.In the pixel portion illustrated in FIG. 15C, the pixel 750 _(—) i, j iselectrically connected to a wiring 754 _(—) i, a wiring 755 _(—) j, anda wiring 756 _(—) j; a pixel 750 _(—) i+1, j is electrically connectedto a wiring 754 _(—) i+1, the wiring 755 _(—) j, and the wiring 756 _(—)j; a pixel 750 _(—) i, j+1 is electrically connected to the wiring 754_(—) i, a wiring 755 _(—) j+1, and a wiring 756 _(—) j+1; a pixel 750_(—) i+1, j+1 is electrically connected to the wiring 754 _(—) i+1, thewiring 755 _(—) j+1, and the wiring 756 _(—) j+1. Note that in the pixelportion illustrated in FIG. 15C, each wiring can be shared between aplurality of pixels in the same column or the same row. Note that in thepixel portion illustrated in FIG. 15C, since the wiring 757 is thecounter electrode and the counter electrode is shared between all thepixels, the wiring 757 is not referred to using the natural number of ior j. Note that in the liquid crystal display device of this embodiment,the structure of the pixel portion illustrated in FIG. 15B can be used.Thus, the wiring 757 can be eliminated from the structure where thewiring 757 is provided, and the wiring 757 can be eliminated if adifferent wiring also serves as the wiring 757, for example.

Note that the pixels in the pixel portion illustrated in FIG. 15C can bedriven by a variety of methods. In particular, by driving the pixels bya method which is referred to as AC drive, deterioration (burn-in) ofthe liquid crystal elements can be suppressed. The operation in the casewhere the pixels in the pixel portion illustrated in FIG. 15C are drivenby AC drive is described with reference to FIG. 15D. FIG. 15D is atiming chart illustrating the operation of the pixels in the pixelportion illustrated in FIG. 15C. Note that here, operation using dotinversion drive, which is one of AC drive, is described as the operationof the pixels in the pixel portion illustrated in FIG. 15C. By using dotinversion drive, flickers which occur in the case of AC drive can besuppressed.

In the pixels in the pixel portion illustrated in FIG. 15C, a switch inthe pixel which is electrically connected to the wiring 755 _(—) j isselected (is in an on state) in a j^(th) gate selection period in oneframe period and is not selected (is in an off state) in the otherperiods. Then, after the j^(th) gate selection period, a (j+1)^(th) gateselection period is provided. By performing sequential scanning in thismanner, all the pixels are sequentially selected in one frame period. Inthe timing chart illustrated in FIG. 15D, for example, the switch in thepixel is selected when the potential is high, and the switch in thepixel is not selected when the potential is low. Note that this exampleis the case where the transistor in each pixel is an n-channeltransistor. In the case of using a p-channel transistor, therelationship between voltage and selection is opposite to that of thecase of using an n-channel transistor.

In the timing chart illustrated in FIG. 15D, a positive potential isapplied to the wiring 754 _(—) i which is used as a signal line and anegative potential is applied to the wiring 754 _(—) i+1 in the j^(th)gate selection period in a k^(th) frame (k is a natural number). Then, anegative potential is applied to the wiring 754 _(—) i and a positivepotential is applied to the wiring 754 _(—) i+1 in the (j+1)^(th) gateselection period in the k^(th) frame. After that, signals whosepolarities are inverted every gate selection period are alternatelyapplied to signal lines. Accordingly, in the k^(th) frame, a positivepotential, a negative potential, a negative potential, and a positivepotential are applied to the pixel 750 _(—) i, j the pixel 750 _(—) i+1,j, the pixel 750 _(—) i, j+1, and the pixel 750 _(—) i+1, j+1,respectively. Then, in a (k+1)^(th) frame, potentials whose polaritiesare opposite to the polarities of the potentials written to the pixelsin the k^(th) frame are written to the pixels as data. Accordingly, inthe (k+1)^(th) frame, a negative potential, a positive potential, apositive potential, and a negative potential are applied to the pixel750 _(—) i, j, the pixel 750 _(—) i+1, j, the pixel 750 _(—) i, j+1, andthe pixel 750 _(—) i+1, j+1, respectively. As described above, a drivingmethod by which potentials whose polarities are opposite to each otherare applied to adjacent pixels in the same frame and the polarities ofpotentials are inverted every one frame in the pixels is dot inversiondrive. By dot inversion drive, deterioration of the liquid crystalelements can be suppressed and flickers viewed when all or part ofimages displayed are uniform can be reduced. Note that voltage which isapplied to all the wirings 756 including the wiring 756 _(—) j and thewiring 756 _(—) j+1 can be made constant voltage. Note that althoughonly the polarity of a potential of the wiring 754 is illustrated in thetiming chart, the level of the potential of the wiring 754 can be avariety of levels in the polarity illustrated in FIG. 15D. Note thatalthough the case where polarities are inverted every one dot (onepixel) is described here, the method of inversion is not limited tothis. Polarities can be inverted every plurality of pixels. For example,by inverting the polarities of potentials which are written in every twogate selection periods, power which is consumed in writing thepotentials can be reduced. Alternatively, polarities can be invertedevery one column (source line inversion) or polarities can be invertedevery one row (gate line inversion).

Note that constant voltage may be applied to the capacitor 753 in thepixel 750 in one frame period. Here, a signal supplied to the wiring 755which is used as a scan line is in a low state in the most part of oneframe period, and substantially constant voltage is applied to thewiring 755. Thus, the other of the terminals of the capacitor 753 in thepixel 750 may be connected to the wiring 755. A structure where theother of the terminals of the capacitor 753 and the wiring 755 areelectrically connected to each other is illustrated in FIG. 15E.

When the structure of a pixel portion illustrated in FIG. 15E iscompared to the structure of the pixel portion illustrated in FIG. 15C,the wiring 756 is eliminated and the one of the terminals of thecapacitor 753 in the pixel 750 and the wiring 755 in the preceding roware electrically connected to each other. Specifically, the one of theterminals of the capacitors 753 in the pixel 750 _(—) i, j+1 and thepixel 750 _(—) i+1, j+1 are electrically connected to the wiring 755_(—) j. The wiring 756 can be eliminated by electrically connecting theone of the terminals of the capacitor 753 in the pixel 750 and thewiring 755 in the preceding row in this manner. Thus, according toreduction in the number of wirings, the aperture ratio of the pixels canbe improved. Note that the one of the terminals of the capacitor 753 maybe connected not to the wiring 755 in the preceding row but to thewiring 755 in a different row. Note that as the driving method of thepixels in the pixel portion illustrated in FIG. 15E, a driving methodwhich is similar to the driving method of the pixels in the pixelportion illustrated in FIG. 15C can be used.

Note that by using the capacitor 753 and the wiring which iselectrically connected to the other of the terminals of the capacitor753, the level of voltage applied to the wiring 754 which is used as thesignal line can be lowered. The structure and the driving method of thepixel portion in this case are described with reference to FIG. 15F andFIG. 15G.

When the structure of the pixel portion illustrated in FIG. 15F iscompared to the structure of the pixel portion illustrated in FIG. 15A,two wirings 756 are provided in one pixel column and electricalconnection to the one of the terminals of the capacitor 753 in the pixel750 is alternately performed in adjacent pixels. Note that the twowirings 756 are referred to as a wiring 756-1 and a wiring 756-2.Specifically, in the range of illustration of FIG. 15F, the one of theterminals of the capacitor 753 in the pixel 750 _(—) i, j iselectrically connected to a wiring 756-1 _(—) j; the one of theterminals of the capacitor 753 in the pixel 750 _(—) i+1, j iselectrically connected to a wiring 756-2 _(—) j; the one of theterminals of the capacitor 753 in the pixel 750 _(—) i, j+1 iselectrically connected to a wiring 756-2 _(—) j+1; the one of theterminals of the capacitor 753 in the pixel 750 _(—) i+1, j+1 iselectrically connected to a wiring 756-1 _(—) j+1.

For example, as illustrated in FIG. 15G, in the case where a positivepotential is written to the pixel 750 _(—) i, j in the k^(th) frame, thewiring 756-1 _(—) j enters into a low state in the j^(th) gate selectionperiod and enters into a high state after the j^(th) gate selectionperiod is finished. Then, the wiring 756-1 _(—) j is kept at the highstate in one frame period, and a negative potential is written in thej^(th) gate selection period in a (k+1)^(th) frame. After that, thewiring 756-1 _(—) j enters into the low state. By changing the polarityof the wiring which is electrically connected to the other of theterminals of the capacitor 753 in a positive direction after thepositive potential is written to the pixel in this manner, the potentialapplied to the liquid crystal element can be changed in the positivedirection by a predetermined level. That is, the level of voltage whichis written to the pixel can be lowered by the predetermined level, sothat power which is consumed in writing signals can be reduced. Notethat in the case where a negative potential is written in the j^(th)gate selection period, by changing the polarity of the wiring which iselectrically connected to the other of the terminals of the capacitor753 in a negative direction after the negative potential is written tothe pixel, the potential applied to the liquid crystal element can bechanged in the negative direction by a predetermined level. Thus, as inthe case of the positive potential, the level of voltage which iswritten to the pixel can be lowered. That is, in the same row of thesame frame, the wirings which are electrically connected to the other ofthe terminals of the capacitor 753 are preferably different wiringsbetween the pixel to which the positive potential is applied and thepixel to which the negative potential is applied.

In the pixel portion illustrated in FIG. 15F, the wiring 756-1 iselectrically connected to the pixel to which the positive potential iswritten in the k^(th) frame, and the wiring 756-2 is electricallyconnected to the pixel to which the negative potential is written in thek^(th) frame. Note that this structure is just an example. For example,in the case of a driving method in which a pixel to which a positivepotential is written and a pixel to which a negative potential iswritten appear every two pixels, it is preferable to perform electricalconnections with the wiring 756-1 and the wiring 756-2 alternately everytwo pixels. Further, in the case where potentials having the samepolarity are written to all the pixels in one row (gate line inversion),one wiring 756 is provided in one row. That is, in the pixel structureof the pixel portion illustrated in FIG. 15C, a driving method by whichthe level of voltage written to a pixel is lowered as described withreference to FIG. 15F and FIG. 15G, can be used.

Next, a pixel structure and a driving method thereof which areparticularly preferable in the case where the operation mode of a liquidcrystal element is a VA (vertical alignment) mode typified by an MVAmode, a PVA mode, or the like are described. The VA mode has advantagesthat a rubbing process is not necessary in manufacturing, the amount oflight leakage is small in displaying black images, and the level ofdrive voltage is low; however, the VA mode has a problem in that thequality of images deteriorates when a screen is viewed from an angle(the viewing angle is narrow). In order to broaden the viewing angle inthe VA mode, it is effective to use a pixel structure where a pluralityof subpixels are provided in one pixel. A pixel structure where aplurality of subpixels are provided in one pixel is described withreference to FIG. 16A and FIG. 16B. FIG. 16A and FIG. 16B are circuitdiagrams each illustrating an example of the structure of a pixel whichcan be used in the liquid crystal display device of this embodiment.

The pixel 750 in the pixel portion of each liquid crystal display deviceillustrated in FIG. 16A and FIG. 16B is an example of the case where twosubpixels (a subpixel 750-1 and a subpixel 750-2) are provided. Notethat the number of subpixels in one pixel is not limited to two, and thenumber of subpixels may be a variety of numbers. As the number ofsubpixels becomes larger, the viewing angle can be further broadened. Aplurality of subpixels can have the same circuit structure. Here, thecase is described in which all the subpixels have a circuit structurewhich is similar to the circuit structure illustrated in FIG. 15A. Notethat the first subpixel 750-1 includes a transistor 751-1, a liquidcrystal element 752-1, and a capacitor 753-1. Connection relationshipsof these elements are similar to the connection relationships in thecircuit structure illustrated in FIG. 15A. In a similar manner, thesecond subpixel 750-2 includes a transistor 751-2, a liquid crystalelement 752-2, and a capacitor 753-2. Connection relationships of theseelements are similar to the connection relationships in the circuitstructure illustrated in FIG. 15A.

In the pixel portion illustrated in FIG. 16A, two wirings 755 which areused as scan lines (a wiring 755-1 and a wiring 755-2) are provided withrespect to two subpixels included in one pixel; one wiring 754 which isused as a signal line is provided; one wiring 756 which is used as acapacitor line is provided. By sharing the signal line and the capacitorline between the two subpixels in this manner, the aperture ratio can beimproved, manufacturing cost can be reduced because the structure of asignal line driver circuit can be simplified, and yield can be improvedbecause the number of connection portions between a liquid crystal paneland a driver circuit can be reduced.

In the pixel portion illustrated in FIG. 16B, one wiring 755 which isused as a scan line is provided with respect to two subpixels includedin one pixel; two wirings 754 (a wiring 754-1 and a wiring 754-2) whichare used as signal lines are provided; one wiring 756 which is used as acapacitor line is provided. By sharing the scan line and the capacitorline between the two subpixels in this manner, the aperture ratio can beimproved and the number of the entire scan lines can be reduced;therefore, the length of each gate line selection period can besufficiently increased even in a high-definition liquid crystal panel,so that appropriate voltage can be written to each pixel.

Next, an example in which the liquid crystal element 752 in the pixelportion illustrated in FIG. 16B is illustrated only by a pixel electrodeof the liquid crystal element 752 and the electrical connection of eachelement is schematically illustrated is described with reference to FIG.16C and FIG. 16D.

In FIG. 16C and FIG. 16D, an electrode 758-1 is a first pixel electrodeand an electrode 758-2 is a second pixel electrode. In FIG. 16C, theelectrode 758-1 corresponds to a second terminal of the liquid crystalelement 752-1 in FIG. 16B, and the electrode 758-2 corresponds to asecond terminal of the liquid crystal element 752-2 in FIG. 16B. Thatis, the electrode 758-1 is electrically connected to one of a sourceterminal and a drain terminal of the transistor 751-1, and the electrode758-2 is electrically connected to one of a source terminal and a drainterminal of the transistor 751-2. On the other hand, in FIG. 16D,connection relationships between the pixel electrodes and thetransistors are opposite to the connection relationships in FIG. 16C.That is, the electrode 758-1 is electrically connected to the one of thesource terminal and the drain terminal of the transistor 751-2, and theelectrode 758-2 is electrically connected to the one of the sourceterminal and the drain terminal of the transistor 751-1.

By alternately arranging the pixels illustrated in FIG. 16C and FIG. 16Din matrix, special advantageous effects can be obtained. An example ofthe structure and the driving method of the pixel portion in this caseis described with reference to FIG. 16E and FIG. 16F. Note that in atiming chart illustrated in FIG. 16F, for example, a switch in the pixelis selected when the potential is high, and the switch in the pixel isnot selected when the potential is low.

In the structure of the pixel portion illustrated in FIG. 16E, thestructure illustrated in FIG. 16C is used for portions which correspondto the pixel 750 _(—) i, j and the pixel 750 _(—) i+1, j+1, and thestructure illustrated in FIG. 16D is used for portions which correspondto the pixel 750 _(—) i+1, j and the pixel 750 _(—) i, j+1. In thisstructure, by performing driving as the timing chart illustrated in FIG.16F, in the j^(th) gate selection period in the k^(th) frame, positivepotentials are written to a first pixel electrode of the pixel 750 _(—)i, j and a second pixel electrode of the pixel 750 _(—) i+1, j, andnegative potentials are written to a second pixel electrode of the pixel750 _(—) i, j and a first pixel electrode of the pixel 750 _(—) i+1, j.In the (j+1)^(th) gate selection period in the k^(th) frame, positivepotentials are applied to a second pixel electrode of the pixel 750 _(—)i, j+1 and a first pixel electrode of the pixel 750 _(—) i+1, j+1, andnegative potentials are applied to a first pixel electrode of the pixel750 _(—) i, j+1 and a second pixel electrode of the pixel 750 _(—) i+1,j+1. In the (k+1)^(th) frame, the polarity of voltage is inverted ineach pixel. Thus, driving which corresponds to dot inversion drive canbe performed in the pixel structure where subpixels are provided and thepolarities of potentials applied to signal lines can be the same in oneframe period. Therefore, power which is consumed in writing data to thepixels can be drastically reduced. Note that voltage which is applied toall the wirings 756 including the wiring 756 _(—) j and the wiring 756_(—) j+1 can be constant voltage.

By using the structure and the driving method illustrated in FIGS. 16Gand 16H, the level of potential which is written to a pixel can belowered. Specifically, capacitor lines which are electrically connectedto a plurality of subpixels included in each pixel are different betweenthe subpixels. That is, by using the structure and the driving methodillustrated in FIGS. 16G and 16H, subpixels to which voltage having thesame polarities are written in the same frame share a capacitor line inthe same row, and subpixels to which voltage having the differentpolarities are written in the same frame use different capacitor linesin the same row. Then, by changing the potentials of the capacitor linesin a positive direction in the subpixels to which voltage having thesame polarities are written and by changing the potentials of thecapacitor lines in a negative direction in the subpixels to whichvoltage having the different polarities are written at the time whenwriting of each row is finished, the level of voltage which is writtento a pixel can be lowered. In specific, two wirings 756 (a wiring 756-1and a wiring 756-2) which are used as capacitor lines are provided ineach row; the first pixel electrode of the pixel 750 _(—) i, j and awiring 756-1 _(—) j are electrically connected to each other through acapacitor; the second pixel electrode of the pixel 750 _(—) i, j and awiring 756-2 _(—) j are electrically connected to each other through acapacitor; a first pixel electrode of the pixel 750 _(—) i+1, j and thewiring 756-1 _(—) j are electrically connected to each other through acapacitor; a second pixel electrode of the pixel 750 _(—) i+1, j and thewiring 756-2 _(—) j are electrically connected to each other through acapacitor; a first pixel electrode of the pixel 750 _(—) i, j+1 and awiring 756-2 _(—) j+1 are electrically connected to each other through acapacitor; a second pixel electrode of the pixel 750 _(—) i, j+1 and awiring 756-1 _(—) j+1 are electrically connected to each other through acapacitor; a first pixel electrode of the pixel 750 _(—) i+1, j+1 andthe wiring 756-2 _(—) j+1 are electrically connected to each otherthrough a capacitor; a second pixel electrode of the pixel 750 _(—) i+1,j+1 and the wiring 756-1 _(—) j+1 are electrically connected to eachother through a capacitor. Note that this structure is just an example.For example, in the case of a driving method in which a pixel to which apositive potential is written and a pixel to which a negative potentialis written appear every two pixels, it is preferable to performelectrical connections with the wiring 756-1 and the wiring 756-2alternately every two pixels. Further, in the case where potentialshaving the same polarity are written to all the pixels in one row (gateline inversion), one wiring 756 is provided in one row. That is, in thestructure of the pixel portion illustrated in FIG. 16E, a driving methodby which the level of voltage written to a pixel is lowered as describedwith reference to FIG. 16G and FIG. 16H, can be used.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 4

In this embodiment, the structure of a transistor which can be used as atransistor included in the driver circuit which is an embodiment of thepresent invention is described.

First, the structure of a transistor which can be used as a transistorincluded in a driver circuit of this embodiment is described withreference to FIGS. 17A and 17B. FIGS. 17A and 17B are cross-sectionalschematic views each illustrating the structure of a transistor whichcan be used in the driver circuit of this embodiment. FIG. 17Aillustrates an example of the structure of a top-gate transistor. FIG.17B illustrates an example of the structure of a bottom-gate transistor.

The transistor illustrated in FIG. 17A includes a substrate 900, asemiconductor layer 902 which is provided over the substrate 900 and hasimpurity regions 901, a gate insulating film 903 which is provided so asto cover the semiconductor layer 902, a gate electrode 904 which isprovided over part of the semiconductor layer 902 with the gateinsulating film 903 interposed therebetween, an interlayer insulatingfilm 906 which is provided over the gate electrode 904 and the gateinsulating film 903 and has opening portions, and a pair of electrodes905 a and 905 b which are provided so as to be in contact with theimpurity regions 901 through the openings.

The transistor illustrated in FIG. 17B includes a substrate 907, a gateelectrode 908 which is provided over the substrate 907, a gateinsulating film 910 which is provided so as to cover the gate electrode908, a semiconductor layer 911 which is provided over portions of thegate insulating film 910, where the gate electrode 908 is not provided,a pair of semiconductor layers 912 a and 912 b which are provided overthe semiconductor layer 911 and have n-type conductivity, an electrode913 a which is provided over one of the pair of semiconductor layers,i.e., the semiconductor layer 912 a, and an electrode 913 b which isprovided over the other of the pair of semiconductor layers, i.e., thesemiconductor layer 912 b.

As each of the substrate 900 and the substrate 907, a glass substrate, aquartz substrate, a silicon substrate, a metal substrate, a stainlesssteel substrate, or the like can be used, for example. Alternatively, aswell as the above substrate, a flexible substrate can be used. Aflexible substrate refers to a substrate which can be bent (isflexible). For example, a plastic substrate or the like formed usingpolycarbonate, polyalylate, polyethersulfone, or the like can be used.Alternatively, as each of the substrate 900 and the substrate 907, anattachment film (formed using polypropylene, polyester, vinyl, polyvinylfluoride, polyvinyl chloride, or the like), paper of a fibrous material,a base material film (polyester, polyamide, an inorganic vapordeposition film, paper, or the like), or the like can be used, forexample.

Each of the semiconductor layer 902 and the semiconductor layer 911 canbe formed using an amorphous semiconductor film, a single crystalsemiconductor film, a polycrystalline semiconductor film, amicrocrystalline (also referred to as microcrystal or semi-amorphous)semiconductor film, or the like, or can be formed by stacking any ofsuch semiconductor films. Alternatively, for each of the semiconductorlayer 902 and the semiconductor layer 911, an oxide semiconductor (e.g.,IGZO (InGaZnO)) can be used. Alternatively, each of the semiconductorlayer 902 and the semiconductor layer 911 can be formed by sputtering,LPCVD, plasma-enhanced CVD, or the like, for example. Alternatively, asemiconductor film having a crystalline structure (a crystallinesemiconductor film) which is obtained by crystallizing an amorphoussemiconductor film by a known technique (a solid phase epitaxy method, alaser crystallization method, a crystallization method using a catalyticmetal, or the like), for example, a polycrystalline silicon film can beused.

As each of the gate insulating film 903 and the gate insulating film910, an insulating nitride film, an insulating oxide film, an insulatingoxide film containing nitrogen, or the like can be used, for example.For example, a silicon oxynitride film, a silicon nitride oxide film, orthe like can be used. Note that a silicon oxynitride film refers to afilm which contains more oxygen than nitrogen and contains oxygen,nitrogen, silicon, and hydrogen at concentrations ranging from 55 to 65atomic percent, 1 to 20 atomic percent, 25 to 35 atomic percent, and 0.1to 10 atomic percent, respectively. Further, a silicon nitride oxidefilm refers to a film which contains more nitrogen than oxygen andcontains oxygen, nitrogen, silicon, and hydrogen at concentrationsranging from 15 to 30 atomic percent, 20 to 35 atomic percent, 25 to 35atomic percent, and 15 to 25 atomic percent, respectively.

As the semiconductor layer 912 a and the semiconductor layer 912 b, asemiconductor layer which has n-type conductivity and containsphosphorus or the like as an impurity element can be used.

For each of the gate electrode 904 and the gate electrode 908, anelement selected from gold, silver, platinum, nickel, silicon, tungsten,chromium, molybdenum, iron, cobalt, copper, palladium, carbon, aluminum,manganese, titanium, tantalum, or the like; or an alloy which contains aplurality of the elements can be used, for example. Alternatively, asingle-layer structure or a layered structure thereof can be used. Asthe alloy which contains a plurality of the elements, an alloy whichcontains aluminum and titanium; an alloy which contains aluminum,titanium, and carbon; an alloy which contains aluminum and nickel; analloy which contains aluminum and carbon; an alloy which containsaluminum, nickel, and carbon; an alloy which contains aluminum andmolybdenum; or the like can be used, for example. Alternatively, alight-transmitting material such as indium tin oxide (ITO), indium tinoxide containing silicon oxide (ITSO), or indium zinc oxide (IZO) can beused. Each of the gate electrode 904 and the gate electrode 908 can beformed by vapor deposition, sputtering, CVD, a printing method, or adroplet discharge method.

As the interlayer insulating film 906, an insulating nitride film, aninsulating oxide film, an insulating oxide film containing nitrogen, orthe like can be used, for example.

Each of the electrode 905 a, the electrode 905 b, the electrode 913 a,and the electrode 913 b serves as a source electrode or a drainelectrode. For each of the electrode 905 a, the electrode 905 b, theelectrode 913 a, and the electrode 913 b, an element selected from gold,silver, platinum, nickel, silicon, tungsten, chromium, molybdenum, iron,cobalt, copper, palladium, carbon, aluminum, manganese, titanium,tantalum, or the like; or an alloy which contains a plurality of theelements can be used, for example. Alternatively, a single-layerstructure or a layered structure thereof can be used. As the alloy whichcontains a plurality of the elements, an alloy which contains aluminumand titanium; an alloy which contains aluminum, titanium, and carbon; analloy which contains aluminum and nickel; an alloy which containsaluminum and carbon; an alloy which contains aluminum, nickel, andcarbon; an alloy which contains aluminum and molybdenum; or the like canbe used, for example. Alternatively, a light-transmitting material suchas indium tin oxide (ITO), indium tin oxide containing silicon oxide(ITSO), or indium zinc oxide (IZO) can be used. Each of the gateelectrode 904 and the gate electrode 908 can be formed by vapordeposition, sputtering, CVD, a printing method, or a droplet dischargemethod. The electrode 905 a, the electrode 905 b, the electrode 913 a,and the electrode 913 b can be formed using different materials. Each ofthe electrode 905 a, the electrode 905 b, the electrode 913 a, and theelectrode 913 b can be formed by vapor deposition, sputtering, CVD, aprinting method, or a droplet discharge method.

As described above, by using any of the above transistors, the drivercircuit which is an embodiment of the present invention can be formed.

Next, a different structure of a bottom-gate transistor is describedwith reference to FIG. 18 as a transistor which can be used in thedriver circuit which is an embodiment of the present invention. FIG. 18is a cross-sectional schematic view illustrating an example of thestructure of a transistor which can be used as a transistor included inthe driver circuit of this embodiment.

The transistor illustrated in FIG. 18 includes a substrate 1000, a gateelectrode 1001 which is provided over the substrate 1000, a gateinsulating film 1002 which is provided so as to cover the gate electrode1001, a microcrystalline semiconductor layer 1003 which is provided overthe gate electrode 1001 with the gate insulating film 1002 interposedtherebetween, a buffer layer 1004 which is provided over themicrocrystalline semiconductor layer 1003, a pair of semiconductorlayers 1005 a and 1005 b which are provided over the buffer layer 1004,an electrode 1006 a which is provided over one of the pair ofsemiconductor layers, i.e., the semiconductor layer 1005 a, and anelectrode 1006 b which is provided over the other of the pair ofsemiconductor layers, i.e., the semiconductor layer 1005 b.

As the substrate 1000, a substrate which can be used as each of thesubstrate 900 and the substrate 907 in FIGS. 17A and 17B can be used.

For the gate electrode 1001, a material and a structure which can beused for each of the gate electrode 904 and the gate electrode 908 inFIGS. 17A and 17B can be used.

For the gate insulating film 1002, a material which can be used for eachof the gate insulating film 903 and the gate insulating film 910 inFIGS. 17A and 17B can be used.

The microcrystalline semiconductor layer 1003 is a layer containing asemiconductor having an intermediate structure between amorphous andcrystalline (including single crystal and polycrystalline) structures.This semiconductor is a semiconductor having a third state, which isstable in terms of free energy, and is a crystalline substance having ashort-range order and lattice distortion, and column-like or needle-likecrystals with a grain size greater than or equal to 0.5 nm and less thanor equal to 50 nm, preferably greater than or equal to 1 nm and lessthan or equal to 20 nm grown in the direction of a normal line withrespect to a surface of the substrate. For the microcrystallinesemiconductor layer 1003, microcrystalline silicon or the like can beused, for example.

The microcrystalline semiconductor layer 1003 has weak n-typeconductivity when an impurity element for controlling valence electronsis not added intentionally. Thus, the threshold voltage Vth ispreferably controlled by adding an impurity element which imparts p-typeconductivity to the microcrystalline semiconductor layer which serves asa channel formation region of a thin film transistor at the same time asor after deposition. A typical example of an impurity element whichimparts p-type conductivity is boron, and an impurity gas such as B₂H₆or BF₃ is preferably mixed into silicon hydride at a proportion higherthan or equal to 1 ppm and lower than or equal to 1000 ppm, preferablyhigher than or equal to 1 ppm and lower than or equal to 100 ppm. Theconcentration of boron is preferably higher than or equal to 1×10¹⁴atoms/cm³ and lower than or equal to 6×10¹⁶ atoms/cm³, for example.

The oxygen concentration of the microcrystalline semiconductor layer1003 is preferably lower than or equal to 1×10¹⁹ cm⁻³, more preferablylower than or equal to 5×10¹⁸ cm⁻³ and each of the nitrogenconcentration and the carbon concentration is preferably lower than orequal to 5×10¹⁸ cm⁻³, more preferably lower than or equal to 1×10¹⁸cm⁻³. By decreasing the concentrations of oxygen, nitrogen, and carbonto be mixed into the microcrystalline semiconductor layer, a channelformation region of the microcrystalline semiconductor layer 1003 can beprevented from being changed into an n-type semiconductor. Further, whenthe concentrations of mixture of these elements are varied amongelements, variations in the threshold voltage Vth occur. Thus, bydecreasing these concentrations, variations in the threshold voltage Vthin the substrate can be reduced.

The carrier mobility of the microcrystalline semiconductor layer 1003 ishigher than the carrier mobility of the buffer layer 1004. Thus, byusing a thin film transistor, a channel formation region of which isformed using a microcrystalline semiconductor, as a transistor in adriver circuit of a display device, the size of the channel formationregion, i.e., the size of the thin film transistor can be decreased.Therefore, the size of the circuit can be decreased, and the frame ofthe display device can be narrowed.

By providing the buffer layer 1004 over the microcrystallinesemiconductor layer 1003, the amount of off-state current of thetransistor can be made smaller than the amount of off-state current inthe case of a single-layer structure of the microcrystallinesemiconductor layer 1003. For the buffer layer 1004, amorphous siliconor the like can be used, for example.

The semiconductor layer 1005 a and the semiconductor layer 1005 b areformed using a semiconductor layer having an impurity element whichimparts n-type or p-type conductivity. As the semiconductor layer havingan impurity element, amorphous silicon or the like can be used, forexample. As the impurity element, phosphorus may be added in the case ofimparting n-type conductivity, and boron may be added in the case ofimparting p-type conductivity. Alternatively, the semiconductor layer1005 a and the semiconductor layer 1005 b can be formed using amicrocrystalline semiconductor material or an amorphous semiconductormaterial. Each of the semiconductor layer 1005 a and the semiconductorlayer 1005 b is preferably formed to a thickness greater than or equalto 2 nm and less than or equal to 50 nm. When each of the semiconductorlayer 1005 a and the semiconductor layer 1005 b is formed to a smallthickness, throughput can be improved.

Each of the electrode 1006 a and the electrode 1006 b serves as a sourceelectrode or a drain electrode. For the electrode 1006 a and theelectrode 1006 b, a material which can be used for the electrode 905 a,the electrode 905 b, the electrode 913 a, and the electrode 913 b inFIGS. 17A and 17B can be used.

Next, a method for manufacturing the transistor illustrated in FIG. 18is described with reference to FIGS. 19A to 19C, FIGS. 20D to 20F, andFIGS. 21G and 21H. FIGS. 19A to 19C, FIGS. 20D to 20F, and FIGS. 21G and21H are cross-sectional schematic views illustrating a method formanufacturing a transistor of this embodiment. Note that as for a thinfilm transistor having a microcrystalline semiconductor layer, ann-channel transistor has higher mobility than a p-channel transistor. Itis preferable that all the thin film transistors formed over the samesubstrate have the same polarity because the number of manufacturingsteps can be reduced. Therefore, in this embodiment, a method formanufacturing an n-channel transistor is described.

First, as illustrated in FIG. 19A, a conductive film 1007 is formed overthe substrate 1000. In this embodiment, a stacked-layer film of analuminum film and a molybdenum film is formed as the conductive film1007. Note that the conductive film 1007 can be formed by sputtering orvacuum vapor deposition.

Next, as illustrated in FIG. 19B, part of the conductive film 1007 isetched, so that the gate electrode 1001 is formed. Specifically, thegate electrode 1001 can be formed in such a way that a resist is formedover the conductive film 1007 by photolithography or an inkjet methodand the conductive film 1007 is selectively etched using the resist as amask. Note that in this step, a scan line (e.g., the scan line 706 inFIG. 13) can be simultaneously formed, for example. Further, the resistis preferably removed after the etching.

End portions of the gate electrode 1001 which is formed by the etchingare preferably tapered. When the end portions are tapered, coverage witha layer which is to be formed over the gate electrode 1001 in a laterstep can be improved.

Next, as illustrated in FIG. 19C, the gate insulating film 1002 isformed so as to cover the gate electrode 1001. The gate insulating film1002 can be formed by CVD, sputtering, or the like. In this embodiment,as an example, the gate insulating film 1002 is formed using astacked-layer film of a nitride film or a nitride oxide film, and anoxide film or an oxynitride film.

A microcrystalline semiconductor film 1008 is formed over the gateinsulating film 1002. The microcrystalline semiconductor film 1008 canbe formed by high-frequency plasma-enhanced CVD with a frequency ofseveral tens to several hundreds of megahertz or a microwaveplasma-enhanced CVD apparatus with a frequency higher than or equal to 1GHz, for example. Plasma which is generated by a microwaveplasma-enhanced CVD apparatus with a frequency higher than or equal to 1GHz has high electron density and many radicals are generated from asource gas and are supplied to the substrate 1000. Thus, radicalreaction on the substrate surface is promoted and the deposition rate ofthe microcrystalline semiconductor film 1008 can be increased. Further,a microwave plasma CVD apparatus which includes a plurality of microwavegenerating apparatuses and a plurality of dielectric plates can generatewide plasma stably. Therefore, a film having high uniformity in filmquality can be formed over a large substrate, and mass productivity(productivity) can be improved. In this embodiment, as an example, thecase where microcrystalline silicon is used for the microcrystallinesemiconductor film is described. A specific method for forming themicrocrystalline semiconductor film 1008 is described below.

For example, the microcrystalline semiconductor film 1008 can be formedby diluting silicon hydride such as SiH₄ or Si₂H₆ with hydrogen or bydiluting silicon hydride with hydrogen and one or plural kinds of raregas elements selected from helium, argon, krypton, or neon. In thatcase, the flow ratio of hydrogen to silicon hydride is 5:1 to 200:1,preferably 50:1 to 150:1, more preferably 100:1. Note that instead ofsilicon hydride, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like can be used.

Note that in the case of forming the microcrystalline semiconductor film1008, crystals of the microcrystalline semiconductor film 1008 grow froma bottom portion of the film toward an upper portion of the film, andneedle-like crystals are formed. This is because crystals grow so as toincrease a crystal surface. However, even if crystals grow in thismanner, the deposition rate of a microcrystalline semiconductor layer isabout 1% to 10% of the deposition rate of an amorphous semiconductorlayer.

Further, in this embodiment, after the microcrystalline semiconductorfilm 1008 is formed, treatment of irradiating the microcrystallinesemiconductor film 1008 with laser light from the surface side of themicrocrystalline semiconductor film 1008 (also referred to as laserprocess (LP) treatment) is preferably performed. The LP treatment isspecifically described below.

In the LP treatment, the microcrystalline semiconductor film 1008 ispreferably irradiated with laser light at energy density such that themicrocrystalline semiconductor film 1008 does not melt. That is, the LPtreatment is laser treatment by which solid-phase crystal growth whichis performed by radiation heating without melting the microcrystallinesemiconductor film 1008 is generated. In other words, the LP treatmentutilizes a critical region in which the deposited microcrystallinesemiconductor film 1008 does not become a liquid phase. Therefore, theLP treatment can also be referred to as critical growth.

The laser light can operate up to an interface between themicrocrystalline semiconductor film 1008 and the gate insulating film1002. Accordingly, when crystals on the surface side of themicrocrystalline semiconductor film 1008 is used as nuclei, solid-phasecrystal growth proceeds from the surface to the interface of the gateinsulating film 1002, and substantially columnar crystals grow.Solid-phase crystal growth by the LP treatment does not increase crystaldiameters but improves the crystallinity in a thickness direction.

In the LP treatment, when a laser beam is condensed in a longrectangular shape (is shaped into a linear laser beam), themicrocrystalline semiconductor film 1008 formed over a glass substratehaving a size of 730 mm×920 mm can be processed by one scanning of thelaser beam, for example. In this case, the LP treatment is performedwith a proportion of overlap of linear laser beams (an overlap ratio) of0% to 90%, preferably 0% to 67%. Thus, the length of treatment time foreach substrate is shortened, so that productivity can be improved. Notethat the shape of a laser beam is not limited to a linear shape, andsimilar treatment can be performed when the shape of a laser beam is aplane shape. Further, the LP treatment is not limited by the size of theglass substrate, and the LP treatment can be used for substrates with avariety of sizes. When the LP treatment is performed, the crystallinityof a region of the interface between the microcrystalline semiconductorfilm 1008 and the gate insulating film 1002 is improved, so thatelectric characteristics of a transistor having a bottom-gate structurecan be improved.

Through such a critical growth, unevenness (convexity called a ridge)generated on a surface of conventional low-temperature polysilicon isnot formed, and the surface of the semiconductor film, on which the LPtreatment is performed, is kept smoothed.

Therefore, the microcrystalline semiconductor film 1008 which isobtained by directly delivering laser light after the deposition hasgrowth mechanism and film quality of a film to be formed, which aregreatly different from those of a microcrystalline semiconductor filmremaining deposited in a conventional technique or a microcrystallinesemiconductor film modified by conduction heating.

Next, as illustrated in FIG. 20D, an amorphous semiconductor film 1009is formed over the microcrystalline film 1008.

The amorphous semiconductor film 1009 can be formed using siliconhydride such as SiH₄ or Si₂H₆ by plasma-enhanced CVD. Alternatively, theamorphous semiconductor film 1009 can be formed by diluting siliconhydride with one or plural kinds of rare gas elements selected fromhelium, argon, krypton, or neon. Alternatively, the amorphoussemiconductor film 1009 containing hydrogen can be formed using hydrogenwith a flow ratio of hydrogen to silicon hydride of 1:1 to 20:1,preferably 1:1 to 10:1, more preferably 1:1 to 5:1. By using siliconhydride, and nitrogen or ammonia, the amorphous semiconductor film 1009containing nitrogen can be formed. Alternatively, by using siliconhydride and a gas containing fluorine, chlorine, bromine, or iodine(e.g., F₂, Cl₂, Br₂, I₂, HF, HCl, HBr; or HI), the amorphoussemiconductor film 1009 containing fluorine, chlorine, bromine, oriodine can be formed. Note that instead of silicon hydride, SiH₂Cl₂,SiHCl₂, SiCl₂, SiF₂, or the like can be used. Note that the thickness ofthe amorphous semiconductor film 1009 is greater than or equal to 100 nmand less than or equal to 500 nm, preferably greater than or equal to150 nm and less than or equal to 400 nm, more preferably greater than orequal to 200 nm and less than or equal to 300 nm. Note that in thiscase, hydrogen is supplied to the microcrystalline semiconductor film1008. That is, by depositing the amorphous semiconductor film 1009 overthe microcrystalline semiconductor film 1008, hydrogen is diffused intothe microcrystalline semiconductor film 1008, so that dangling bonds canbe terminated.

Alternatively, the amorphous semiconductor film 1009 can be formed bysputtering an amorphous semiconductor which is used as a target inhydrogen or a rare gas. In this case, when ammonia, nitrogen, or N₂O iscontained in an atmosphere, an amorphous semiconductor film containingnitrogen can be formed. Alternatively, when a gas containing fluorine,chlorine, bromine or iodine (F₂, Cl₂, Br₂, I₂, HF, HCl, HBr, HI or thelike) is contained in the atmosphere, an amorphous semiconductor layerincluding fluorine, chlorine, bromine or iodine (e.g., F₂, Cl₂, Br₂, I₂,HF, HCl, HBr, or HI), an amorphous semiconductor film containingfluorine, chlorine, bromine, or iodine can be formed.

Alternatively, after forming the amorphous semiconductor film 1009,hydrogenation, nitridation, or halogenation of a surface of theamorphous semiconductor film 1009 may be performed through processing ofthe surface of the amorphous semiconductor film 1009 with hydrogenplasma, nitrogen plasma, or halogen plasma. Alternatively, the surfaceof the amorphous semiconductor film 1009 may be processed with heliumplasma, neon plasma, argon plasma, krypton plasma, or the like.

It is preferable that the amorphous semiconductor film 1009 does notcontain crystal grains. Therefore, when the amorphous semiconductor film1009 is formed by high-frequency plasma-enhanced CVD with a frequency ofseveral tens to several hundreds of megahertz or microwaveplasma-enhanced CVD, deposition conditions are preferably controlled sothat the amorphous semiconductor film 1009 does not contain crystalgrains.

Note that the amorphous semiconductor film 1009 is formed so that animpurity which imparts one conductivity type, such as phosphorus orboron, is not contained. In particular, it is preferable that boron orphosphorus added to the microcrystalline semiconductor film 1008 inorder to control the threshold voltage be not added to the amorphoussemiconductor film 1009. For example, in the case where the amorphoussemiconductor film 1009 contains phosphorus, a PN junction is formedbetween the microcrystalline semiconductor layer 1003 and the amorphoussemiconductor film 1009. Alternatively, in the case where the amorphoussemiconductor film 1009 contains boron, a PN junction is formed betweenthe amorphous semiconductor film 1009, and the semiconductor layer 1005a and the semiconductor layer 1005 b. Alternatively, in the case wherethe amorphous semiconductor 1009 contains both boron and phosphorus, arecombination center is generated, which causes leakage current. Whenthe amorphous semiconductor 1009 does not contain such an impurity whichimparts one conductivity type, a region where leakage current isgenerated is not provided, so that leakage current can be reduced.Further, when the amorphous semiconductor film 1009 to which an impuritywhich imparts one conductivity type, such as phosphorus or boron, is notadded is provided between the semiconductor layer 1005 a and thesemiconductor layer 1005 b, and the microcrystalline semiconductor layer1003, diffusion of impurities contained in the microcrystallinesemiconductor layer 1003 which serves as a channel formation region andthe semiconductor layer 1005 a and the semiconductor layer 1005 b whichserve as part of a source region and a drain region can be prevented.

In addition, a semiconductor film 1010 is formed over the amorphoussemiconductor film 1009. In the case of using an impurity element whichimparts n-type conductivity, phosphorus or the like is added, forexample. In the case of adding phosphorus, phosphorus can be added byadding a gas such as PH₃ to silicon hydride. Alternatively, in the caseof using an impurity element which imparts p-type conductivity, boron orthe like is added, for example. In the case of adding boron, boron canbe added by adding a gas such as B₂H₆ to silicon hydride.

Note that in this embodiment, it is preferable that the gate insulatingfilm 1002, the microcrystalline semiconductor film 1008, and theamorphous semiconductor film 1009 be successively formed. Morepreferably, the gate insulating film 1002, the microcrystallinesemiconductor film 1008, the amorphous semiconductor film 1009, and thesemiconductor film 1010 are successively formed. By successiveformation, since each film is not exposed to the atmosphere, eachinterface of stacked layers can be formed without being contaminated byan atmospheric constituent or a contaminant impurity element floating inthe atmosphere. Thus, variations in electric characteristics of thinfilm transistors formed using these films can be reduced, and a drivercircuit having high reliability can be manufactured with high yield.

Next, the microcrystalline semiconductor film 1008, the amorphoussemiconductor film 1009, and the semiconductor film 1010 are selectivelyetched.

Specifically, first, a resist is formed on part of the semiconductorfilm 1010. For example, the resist is formed by photolithography, aninkjet method, or the like.

Next, the microcrystalline semiconductor film 1008, the amorphoussemiconductor film 1009, and the semiconductor film 1010 are selectivelyetched by using the resist as a mask. In this case, the microcrystallinesemiconductor layer 1003 is formed by the etching, as illustrated inFIG. 20E. Note that the resist is preferably removed after the etching.

Note that the etching is performed so that end portions of a layer wherethe microcrystalline semiconductor film, the amorphous semiconductorfilm, and an impurity semiconductor film are stacked are tapered. Thetaper angle is greater than or equal to 30° and less than or equal to90°, preferably greater than or equal to 40° and less than or equal to80°. When the etching is performed so that the end portions are tapered,the semiconductor film 1010 and the microcrystalline semiconductor film1008 can be prevented from being directly in contact with each other.Further, a distance between the layers at the end portions can besufficiently ensured, so that leakage current at the end portions can bereduced.

In addition, when the end portions are tapered, coverage with a layerwhich is to be formed thereover in a later step can be improved.

Next, as illustrated in FIG. 20F, a conductive film 1011 is formed overthe semiconductor film 1010.

For example, the conductive film 1011 can be formed by sputtering,vacuum vapor deposition, or the like. Alternatively, the conductive film1011 can be formed by discharging a conductive nanopaste of silver,gold, copper, or the like by a screen printing method, an inkjet method,or the like and baking the conductive nanopaste.

Next, the conductive film 1011 is etched. Specifically, first, a resistis selectively formed over the conductive film 1011. Then, theconductive film 1011 is etched using the resist as a mask. In this case,the pair of electrodes 1006 a and 1006 b are formed, as illustrated inFIG. 21G.

Next, the semiconductor film 1010 and the amorphous semiconductor film1009 are etched. By the etching, the buffer layer 1004 and the pair ofsemiconductor layers 1005 a and 1005 b are formed, as illustrated inFIG. 21H.

In this case, part of the buffer layer 1004 is etched, so that adepression is formed. The buffer layer 1004 is preferably formed to athickness such that part of the amorphous semiconductor film 1009, whichoverlaps with the depression, remains. It is preferable that thethickness of a remaining portion (a portion overlapping with thedepression) after the etching be approximately half the thickness beforethe etching. Note that the thickness before the etching is greater thanor equal to 100 nm and less than or equal to 500 nm as described above,preferably greater than or equal to 150 nm and less than or equal to 400nm, more preferably greater than or equal to 200 nm and less than orequal to 300 nm. The buffer layer 1004 serves as an etching stopper forthe microcrystalline semiconductor layer 1003.

In this embodiment, a structure where end portions of the electrode 1006a and the electrode 1006 b are not aligned with end portions of thesemiconductor layer 1005 a and the semiconductor layer 1005 b can beused. Thus, a distance between the end portions of the electrode 1006 aand the electrode 1006 b is increased, so that a distance between one ofthe source electrode and the drain electrode and the other of the sourceelectrode and the drain electrode is sufficiently large. Thus, leakagecurrent can be reduced and short-circuit can be prevented. Further,since the end portions of the electrode 1006 a and the electrode 1006 bare not aligned with the end portions of the semiconductor layer 1005 aand the semiconductor layer 1005 b, an electric field does not easilyconcentrate on the end portions of the electrode 1006 a and theelectrode 1006 b and the end portions of the semiconductor layer 1005 aand the semiconductor layer 1005 b. Therefore, a thin film transistorwhich has high reliability, small off-state current, and high withstandvoltage can be formed.

Through the above steps, the thin film transistor illustrated in FIG. 18can be manufactured.

Further, as illustrated in FIG. 18, a transistor having amicrocrystalline semiconductor layer has higher reliability than atransistor having only an amorphous semiconductor layer. Thus, by usingthe transistor having a microcrystalline semiconductor layer in thedriver circuit which is an embodiment of the present invention,malfunctions can be suppressed.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 5

In this embodiment, electronic devices each using a display device whichis an embodiment of the present invention for a display portion aredescribed.

The display device which is an embodiment of the present invention canbe used for display portions of a variety of electronic devices.Examples of electronic devices for which the display device which is anembodiment of the present invention can be used are cameras such asvideo cameras and digital cameras, goggle-type displays (head-mounteddisplays), navigation systems, audio reproducing devices (e.g., caraudio equipment or audio component sets), laptops, game machines, mobilephones, portable information terminals (including mobile computers,mobile music players, a portable game machines, e-book readers, anddevices which incorporate computers and have a plurality of functions byperforming different kinds of data processing), image reproducingdevices provided with recording media (specifically devices whichreproduce the content of recording media such as DVDs (digital versatiledisc) and have displays for displaying the reproduced images), and thelike. Specific examples of such electronic devices are described withreference to FIGS. 22A to 22H and FIGS. 23A to 23C. FIGS. 22A to 22H andFIGS. 23A to 23C each illustrate an example of the structure of anelectronic device of this embodiment.

FIG. 22A illustrates a display device, which includes a housing 1101, asupport base 1102, a display portion 1103, speaker portions 1104, videoinput terminals 1105, and the like. The display device which is anembodiment of the present invention can be used for the display portion1103. Note that the display device includes all display devices such asdisplay devices for personal computers, for receiving TV broadcast, andfor displaying advertisements, in its category.

FIG. 22B illustrates a digital still camera, which includes a main body1111, a display portion 1112, an image receiving portion 1113, operationkeys 1114, an external connection port 1115, a shutter button 1116, andthe like. The display device which is an embodiment of the presentinvention can be used for the display portion 1112.

FIG. 22C illustrates a laptop, which includes a main body 1121, ahousing 1122, a display portion 1123, a keyboard 1124, an externalconnection port 1125, a pointing device 1126, and the like. The displaydevice which is an embodiment of the present invention can be used forthe display portion 1123.

FIG. 22D illustrates a mobile computer, which includes a main body 1131,a display portion 1132, a switch 1133, operation keys 1134, an infraredport 1135, and the like. The display device which is an embodiment ofthe present invention can be used for the display portion 1132.

FIG. 22E illustrates a portable image reproducing device provided with arecording medium (specifically a DVD player), which includes a main body1141, a housing 1142, a display portion A 1143, a display portion B1144, a recording medium (e.g., a DVD) reading portion 1145, operationkeys 1146, a speaker portion 1147, and the like. The display portion A1143 mainly displays image data, and the display portion B 1144 mainlydisplays text data. The display device which is an embodiment of thepresent invention can be used for each of the display portion A 1143 andthe display portion B 1144. Note that the image reproducing deviceprovided with a recording medium includes a home-use game machine andthe like in its category.

FIG. 22F illustrates a goggle-type display (a head-mounted display),which includes a main body 1151, a display portion 1152, and an armportion 1153. The display device which is an embodiment of the presentinvention can be used for the display portion 1152.

FIG. 22G illustrates a video camera, which includes a main body 1161, adisplay portion 1162, a housing 1163, an external connection port 1164,a remote control receiving portion 1165, an image receiving portion1166, a battery 1167, an audio input portion 1168, operation keys 1169,and the like. The display device which is an embodiment of the presentinvention can be used for the display portion 1162.

FIG. 22H illustrates a mobile phone, which includes a main body 1171, ahousing 1172, a display portion 1173, an audio input portion 1174, anaudio output portion 1175, operation keys 1176, an external connectionport 1177, an antenna 1178, and the like. The display device which is anembodiment of the present invention can be used for the display portion1173. Note that the display portion 1173 displays white text on a blackscreen, so that current consumption of the mobile phone can besuppressed.

FIGS. 23A to 23C illustrate an example of a portable informationterminal having a plurality of functions. FIG. 23A is a front view ofthe portable information terminal; FIG. 23B is a rear view of theportable information terminal; FIG. 23C is a development view of theportable information terminal. A portable information terminal whoseexample is illustrated in FIGS. 23A to 23C can have a plurality offunctions. For example, in addition to a telephone function, such aportable information terminal can have a function of processing avariety of pieces of data by incorporating a computer.

The portable information terminal illustrated in FIGS. 23A to 23Cincludes two housings 1180 and 1181. The housing 1180 includes a displayportion 1182, a speaker 1183, a microphone 1184, operation keys 1185, apointing device 1186, a camera lens 1187, an external connectionterminal 1188, an earphone terminal 1189, and the like. The housing 1181includes a keyboard 1190, an external memory slot 1191, a camera lens1192, a light 1193, and the like. In addition, an antenna isincorporated in the housing 1181.

Further, in addition to the above structure, a contactless IC chip, asmall memory device, or the like may be incorporated.

The display device which is an embodiment of the present invention canbe used for the display portion 1182 and a display direction changes asappropriate depending on the usage. Since the camera lens 1187 isprovided on the same plane as the display portion 1182, videophone ispossible. Further, still images and moving images can be taken with thecamera lens 1192 and the light 1193 with the display portion 1182 usedas a viewfinder. The speaker 1183 and the microphone 1184 can be usedfor videophone calls, recording, and playing sound, and the like as wellas voice calls. With the operation keys 1185, operation of incoming andoutgoing calls, simple information input for e-mail or the like,scrolling of a screen, cursor motion, and the like are possible.Further, the housings 1180 and 1181 which overlap with each other (FIG.23A) can slide to be developed as illustrated in FIG. 23C so as to beused as the portable information terminal. In this case, smoothoperation is possible by using the keyboard 1190 and the pointing device1186. The external connection terminal 1188 can be connected to an ACadapter and a variety of cables such as a USB cable and can performstoring electricity and data communication with a personal computer orthe like. Furthermore, a large amount of data can be stored and moved byinserting a recording medium into the external memory slot 1191.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

As described above, the display device which is an embodiment of thepresent invention can be used for the display portion of a variety ofelectronic devices as above.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

This application is based on Japanese Patent Application serial no.2008-157400 filed with Japan Patent Office on Jun. 17, 2008, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A driver circuit comprising: a first transistor; asecond transistor; a third transistor; a fourth transistor; a fifthtransistor; a sixth transistor; a seventh transistor; an eighthtransistor; and a capacitor, wherein each of the first to eighthtransistors comprises a gate terminal, a source terminal, and a drainterminal, wherein one of the source terminal and the drain terminal ofthe first transistor is electrically connected to one of the sourceterminal and the drain terminal of the second transistor, one of thesource terminal and the drain terminal of the third transistor, one ofthe source terminal and the drain terminal of the fourth transistor, andthe gate terminal of the fifth transistor, wherein the gate terminal ofthe third transistor is electrically connected to the gate terminal ofthe sixth transistor, wherein one of the source terminal and the drainterminal of the fifth transistor is electrically connected to one of thesource terminal and the drain terminal of the sixth transistor and iselectrically connected to one of the source terminal and the drainterminal of the seventh transistor, wherein the gate terminal of theeighth transistor is electrically connected to the other one of thesource terminal and the drain terminal of the first transistor and thegate terminal of the first transistor, wherein one of the sourceterminal and the drain terminal of the eighth transistor is electricallyconnected to the gate terminal of the fourth transistor, and wherein thecapacitor is provided between the gate terminal of the fifth transistorand the one of the source terminal and the drain terminal of the fifthtransistor.
 3. The driver circuit according to claim 2, wherein the gateterminal of the second transistor is electrically connected to a firstline, the gate terminal of the fourth transistor is electricallyconnected to a second line, the gate terminal of the seventh transistoris electrically connected to a third line, and the one of the sourceterminal and the drain terminal of the seventh transistor iselectrically connected to a fourth line, and wherein the first line, thesecond line, the third line, and the fourth line are different lines. 4.The driver circuit according to claim 2, wherein the other one of thesource terminal and the drain terminal of the fifth transistor isconfigured to be supplied with a clock signal, and wherein the gateterminal of the seventh transistor is configured to be supplied with aclock signal.
 5. The driver circuit according to claim 2, wherein allthe first to eighth transistors are n-channel transistors.
 6. The drivercircuit according to claim 2, further comprising a first capacitor and asecond capacitor, wherein the gate terminal of the third transistor iselectrically connected to the first capacitor, and wherein the gateterminal of the fourth transistor is electrically connected to thesecond capacitor.
 7. The driver circuit according to claim 2, whereinthe other one of the source terminal and the drain terminal of thesecond transistor, the other one of the source terminal and the drainterminal of the third transistor, the other one of the source terminaland the drain terminal of the fourth transistor, the other one of thesource terminal and the drain terminal of the sixth transistor, theother one of the source terminal and the drain terminal of the seventhtransistor, and the other one of the source terminal and the drainterminal of the eighth transistor are electrically connected to a lineconfigured to be supplied with a voltage.
 8. A liquid crystal displaydevice comprising: a pixel portion comprising: a liquid crystal element;and a transistor configured to control the liquid crystal element; and adriver circuit configured to control to the pixel portion, the drivercircuit comprising: a first transistor; a second transistor; a thirdtransistor; a fourth transistor; a fifth transistor; a sixth transistor;a seventh transistor; an eighth transistor; and a capacitor, whereineach of the first to eighth transistors comprises a gate terminal, asource terminal, and a drain terminal, wherein one of the sourceterminal and the drain terminal of the first transistor is electricallyconnected to one of the source terminal and the drain terminal of thesecond transistor, one of the source terminal and the drain terminal ofthe third transistor, one of the source terminal and the drain terminalof the fourth transistor, and the gate terminal of the fifth transistor,wherein the gate terminal of the third transistor is electricallyconnected to the gate terminal of the sixth transistor, wherein one ofthe source terminal and the drain terminal of the fifth transistor iselectrically connected to one of the source terminal and the drainterminal of the sixth transistor and is electrically connected to one ofthe source terminal and the drain terminal of the seventh transistor,wherein the gate terminal of the eighth transistor is electricallyconnected to the other one of the source terminal and the drain terminalof the first transistor and the gate terminal of the first transistor,wherein one of the source terminal and the drain terminal of the eighthtransistor is electrically connected to the gate terminal of the fourthtransistor, and wherein the capacitor is provided between the gateterminal of the fifth transistor and the one of the source terminal andthe drain terminal of the fifth transistor.
 9. The liquid crystaldisplay device according to claim 8, wherein the gate terminal of thesecond transistor is electrically connected to a first line, the gateterminal of the fourth transistor is electrically connected to a secondline, the gate terminal of the seventh transistor is electricallyconnected to a third line, and the one of the source terminal and thedrain terminal of the seventh transistor is electrically connected to afourth line, and wherein the first line, the second line, the thirdline, and the fourth line are different lines.
 10. The liquid crystaldisplay device according to claim 8, wherein the other one of the sourceterminal and the drain terminal of the fifth transistor is configured tobe supplied with a clock signal, and wherein the gate terminal of theseventh transistor is configured to be supplied with a clock signal. 11.The liquid crystal display device according to claim 8, wherein all thefirst to eighth transistors are n-channel transistors.
 12. The liquidcrystal display device according to claim 8, further comprising a firstcapacitor and a second capacitor, wherein the gate terminal of the thirdtransistor is electrically connected to the first capacitor, and whereinthe gate terminal of the fourth transistor is electrically connected tothe second capacitor.
 13. The liquid crystal display device according toclaim 8, wherein the other one of the source terminal and the drainterminal of the second transistor, the other one of the source terminaland the drain terminal of the third transistor, the other one of thesource terminal and the drain terminal of the fourth transistor, theother one of the source terminal and the drain terminal of the sixthtransistor, the other one of the source terminal and the drain terminalof the seventh transistor, and the other one of the source terminal andthe drain terminal of the eighth transistor are electrically connectedto a line configured to be supplied with a voltage.
 14. A liquid crystaldisplay device comprising: a pixel portion comprising: a liquid crystalelement; and a transistor configured to control the liquid crystalelement; and a gate driver circuit configured to control to the pixelportion, the gate driver circuit comprising: a first transistor; asecond transistor; a third transistor; a fourth transistor; a fifthtransistor; a sixth transistor; a seventh transistor; an eighthtransistor; and a capacitor, wherein each of the first to eighthtransistors comprises a gate terminal, a source terminal, and a drainterminal, wherein one of the source terminal and the drain terminal ofthe first transistor is directly connected to one of the source terminaland the drain terminal of the second transistor, one of the sourceterminal and the drain terminal of the third transistor, one of thesource terminal and the drain terminal of the fourth transistor, and thegate terminal of the fifth transistor, wherein the gate terminal of thethird transistor is directly connected to the gate terminal of the sixthtransistor, wherein one of the source terminal and the drain terminal ofthe fifth transistor is directly connected to one of the source terminaland the drain terminal of the sixth transistor and is directly connectedto one of the source terminal and the drain terminal of the seventhtransistor, wherein the gate terminal of the eighth transistor isdirectly connected to the other one of the source terminal and the drainterminal of the first transistor and the gate terminal of the firsttransistor, wherein one of the source terminal and the drain terminal ofthe eighth transistor is directly connected to the gate terminal of thefourth transistor, wherein the capacitor is provided between the gateterminal of the fifth transistor and the one of the source terminal andthe drain terminal of the fifth transistor, and wherein at least one ofthe first to eighth transistors comprises a channel formation regioncomprising an oxide semiconductor.
 15. The liquid crystal display deviceaccording to claim 14, wherein the gate terminal of the secondtransistor is directly connected to a first line, the gate terminal ofthe fourth transistor is directly connected to a second line, the gateterminal of the seventh transistor is directly connected to a thirdline, and the one of the source terminal and the drain terminal of theseventh transistor is directly connected to a fourth line, and whereinthe first line, the second line, the third line, and the fourth line aredifferent lines.
 16. The liquid crystal display device according toclaim 14, wherein the other one of the source terminal and the drainterminal of the fifth transistor is configured to be supplied with aclock signal, and wherein the gate terminal of the seventh transistor isconfigured to be supplied with a clock signal.
 17. The liquid crystaldisplay device according to claim 14, wherein all the first to eighthtransistors are n-channel transistors.
 18. The liquid crystal displaydevice according to claim 14, further comprising a first capacitor and asecond capacitor, wherein the gate terminal of the third transistor isdirectly connected to the first capacitor, and wherein the gate terminalof the fourth transistor is directly connected to the second capacitor.19. The liquid crystal display device according to claim 14, wherein theother one of the source terminal and the drain terminal of the secondtransistor, the other one of the source terminal and the drain terminalof the third transistor, the other one of the source terminal and thedrain terminal of the fourth transistor, the other one of the sourceterminal and the drain terminal of the sixth transistor, the other oneof the source terminal and the drain terminal of the seventh transistor,and the other one of the source terminal and the drain terminal of theeighth transistor are directly connected to a line configured to besupplied with a voltage.